# Scrabble Game — Architecture

Source of truth for the platform architecture, transport, security model and
cross-service contracts. User-visible behaviour per domain lives in
[`FUNCTIONAL.md`](FUNCTIONAL.md); the staged build order lives in
[`../PLAN.md`](../PLAN.md). This document always describes the **current**
design, not the history of how it was reached. Sections describing
not-yet-implemented components are marked *(planned)*.

## 1. Overview

Three executables plus per-platform side-services:

- **`gateway`** — the only public ingress (module `scrabble/gateway`). Performs
  anti-abuse (rate limiting), authenticates the player against the originating
  platform (or an email/guest session), resolves the internal `user_id`, and
  forwards authenticated traffic to `backend` with an `X-User-ID` header. Serves the
  backend's admin console at `/_gm` on its public listener behind HTTP Basic Auth.
  Bridges live events from `backend` to the
  client. The shared wire contracts (the push proto and the FlatBuffers edge
  payloads) live in `scrabble/pkg`, imported by both `gateway` and `backend`.
- **`backend`** — internal-only service that owns every domain concern:
  identity/sessions, accounts and linking, lobby and matchmaking, the game
  runtime, the robot opponent, chat, notifications, statistics, history, and
  administration. Embeds the **`scrabble-solver`** engine **as a library,
  in-process** — there is no per-game container. The only network consumer of
  `backend` is `gateway` (plus platform side-services over an internal API).
- **`ui`** — pure-HTML5 client (plain Svelte 5 + TypeScript + Vite, static build;
  no SvelteKit). Talks to `backend` only through `gateway` over Connect-RPC +
  FlatBuffers, with the edge TS bindings generated from the **same** `edge.proto`
  and `scrabble.fbs` and committed under `ui/src/gen/`. The **playable slice**
  (Stage 7) covers auth, "my games", auto-match, the board (play/pass/exchange/
  resign), hint, word-check, chat/nudge, the live stream, i18n (en/ru) and a profile
  view; the social/account/history surfaces follow in Stage 8. There is no board on
  the wire — the client **reconstructs the 15×15 board by replaying the move
  journal** (§9.1) and renders board, tiles, premium squares and effects as pure
  CSS + Unicode (no image/font/SVG assets). Tiles are placed by Pointer-Events drag
  or tap; a CSS-token theme is light/dark and Telegram-themeParams-ready; navigation
  is a hash router and the session token is held in memory + IndexedDB. A build-flagged
  in-memory mock transport (`pnpm start`) runs the whole slice with no backend.
  Embeddable in platform webviews; packageable to native (iOS/Android) via Capacitor.
  The client uses a mobile-app shell (a growing nav bar; content pinned to the bottom),
  a one-line **announcement banner** under the nav (a client-side mock rotation today —
  a server-driven channel later, §10), and a client **board-style** setting (bonus-label
  mode). The visual/interaction design system is documented in
  [`UI_DESIGN.md`](UI_DESIGN.md).
- **`platform/telegram`** — the Telegram side-service (the "connector", module
  `scrabble/platform/telegram`). It is the only component holding the bot token: it
  runs the Bot API long-poll loop (Mini App launch + `/start` deep-links) and serves
  a gRPC API (`pkg/proto/telegram/v1`) that `gateway` (Mini App initData validation
  and out-of-app push) and `backend` (operator broadcasts) call over the
  trusted internal network. Its generic delivery methods are **platform-agnostic**
  (keyed by the identity `external_id`), so a future VK/MAX connector reuses them; only
  initData validation is Telegram-specific. It runs in its own container, egressing to
  Telegram through a VPN sidecar.

```mermaid
flowchart LR
  Client((Client / webview)) -- Connect-RPC + FlatBuffers (h2c) --> Gateway
  Gateway -- REST/JSON, X-User-ID --> Backend
  Backend -- gRPC server-stream (live events) --> Gateway
  Gateway -- in-app stream --> Client
  Backend -- pgx --> Postgres[(Postgres)]
  Backend -. embeds .- Solver[[scrabble-solver library]]
  Gateway -- gRPC (validate initData, out-of-app push) --> Telegram[Telegram connector]
  Backend -. operator broadcasts (gRPC) .-> Telegram
  Telegram -- Bot API (via VPN sidecar) --> TgCloud((Telegram))
```

The MVP runs `gateway` and `backend` as single-instance processes inside a
trusted network. No Redis is planned (anti-replay crypto was deliberately
dropped). Horizontal scaling is explicit future work.

## 2. Transport

- **client ↔ gateway**: **Connect-RPC + FlatBuffers** over HTTP/2 cleartext
  (`h2c`). Binary payloads, server-streaming for the in-app live channel,
  first-class JS clients (`@connectrpc/connect-web` + the `flatbuffers` npm
  package). The contract is kept minimal: a single `Gateway` service (defined in
  `gateway/proto/edge/v1`) with `Execute(message_type, payload, request_id)` for
  unary operations and `Subscribe` for the live stream. The proto envelope is a
  thin carrier; the real request/response and event bodies are **FlatBuffers**
  tables (`pkg/fbs`, the `scrabblefb` namespace) inside the `payload` bytes, which
  the gateway transcodes to and from the backend's JSON. The session token rides
  in the `Authorization: Bearer` header (there is no per-request signing, §3);
  auth operations are unauthenticated and return the minted token. A unary
  operation's domain outcome rides back in `ExecuteResponse.result_code` (HTTP
  200); only edge failures (rate limit, missing session, unknown type, internal)
  surface as Connect error codes.
- **Alphabet on the wire (Stage 13)**: live play exchanges **alphabet indices**, not
  concrete letters. The rack (`StateView.rack`), the `SubmitPlay`/`Evaluate` tiles, the
  `Exchange` tiles and the `CheckWord` word are `ubyte` indices into the variant's alphabet
  (a blank is the sentinel index **255**). The client is **alphabet-agnostic**: on a
  per-variant cache miss it sets `StateRequest.include_alphabet`, and the backend embeds the
  variant's `(index, letter, value)` table (`engine.AlphabetTable`, derived from the solver
  ruleset — no dictionary) for display; the client caches it by variant and renders the rack
  and the blank chooser from it. The backend maps index↔letter at its REST edge, so the
  gateway forwards indices **verbatim** (it holds no alphabet table) and the engine's
  letter-based domain API — shared with the robot — is unchanged. The table is pinned by the
  solver version, so it cannot drift from the running backend. The **move journal, history
  and GCG are unaffected** (they stay decoded concrete characters, §9.1).
- **gateway ↔ backend (sync)**: plain HTTP REST/JSON. The gateway injects
  `X-User-ID` for authenticated requests; `backend` never re-derives identity
  from the body.
- **backend → gateway (live)**: a single gRPC server-stream carries live events
  (your-turn, opponent-moved, chat, nudge). The gateway bridges them to the
  client's in-app stream while the app is open. Out-of-app delivery uses
  platform-native push via the platform side-service.

## 3. Authentication & sessions

Platform-native, deliberately simple: **no Ed25519 client keys, no per-request
signing, no anti-replay crypto** (these were considered and dropped — players
arrive from a platform rather than completing a mandatory registration).

- The gateway validates the originating credential **once** — Telegram `initData`
  (delegated to the connector's `ValidateInitData` RPC, which holds the bot token —
  the HMAC secret — so it never reaches the gateway), an email-code login, or a guest
  bootstrap — then mints a **thin opaque server session token** (`session_id`). First
  Telegram contact seeds the new account's language (from the launch `language_code`)
  and display name (§4).
- The client holds `session_id` in memory for the app session (browser/OS
  storage is optional and may be unavailable; losing it means re-login).
- The gateway caches `session → user_id` and injects `X-User-ID`. Session
  records live in `backend`, which stores only a **SHA-256 hash** of the opaque
  token (never the plaintext), keeps a warmed in-memory cache for fast
  resolution, and treats sessions as **revoke-only** — they have no TTL and live
  until explicitly revoked (`status` → `revoked`). A revoke can target one token or,
  on an account merge (§4), **every** session of the retired account
  (`RevokeAllForAccount`, which also evicts them from the warm cache).
- **Guest** = ephemeral web session (no platform, no email). A guest is backed by
  a durable `accounts` row flagged `is_guest` and carrying **no identity** — the
  row is a technical necessity (the `sessions` and `game_players` foreign keys
  require one, the same way the robot pool is durable), not a profile: no
  friends, statistics or history are kept for it, and it is restricted to
  auto-match. Platform and email users are auto-provisioned **durable** accounts
  with an identity. (Reaping abandoned guest rows is deferred — PLAN.md TODO-3.)

## 4. Accounts, identities, linking & merge

- One internal account may carry several **platform identities**
  (`telegram`, `vk`, …) plus an optional **email** identity. First contact from
  a platform auto-provisions a durable account bound to that platform identity.
  Concretely, platform and email identities share one `identities` table keyed by
  a unique `(kind, external_id)`; email is an identity with `kind=email` and a
  `confirmed` flag. A synthetic `kind='robot'` identity (Stage 5) backs each pooled
  robot opponent (§7). The **email confirm-code flow** (Stage 4) binds an email to the
  authenticated account: a 6-digit code (stored only as a SHA-256 hash, 15-minute
  TTL, ≤ 5 attempts) is sent through a `Mailer` seam (an SMTP relay, or a
  development log mailer when none is configured) and, once verified, attaches a
  confirmed email identity. Accounts and identities use application-generated
  **UUIDv7** primary keys. A service flag `paid_account` (lifetime one-time
  payment; no purchase flow yet) is carried on the account and ORed on a merge.
- **Linking** (Stage 11) is initiated from an authenticated profile and proves
  control of the identity before attaching it: **email** through the confirm-code
  flow, **Telegram** through the web **Login Widget** (validated by the connector,
  HMAC under `SHA-256(bot_token)` — distinct from Mini App initData; the gateway
  passes the trusted `external_id` to the backend, as for `auth.telegram`). The
  request step **always** sends/accepts the proof (no pre-send "already taken"
  signal, so a probe cannot enumerate registered addresses); a required **merge**
  is revealed **only after** the proof is verified and is performed behind an
  explicit, irreversible confirmation. A free identity is simply attached (and a
  guest is promoted to durable, clearing `is_guest`).
- **Merge** retires the account that owns the linked identity into the **current**
  account, in a single transaction (`internal/accountmerge`): statistics summed
  (max points kept), the hint wallet summed, `paid_account` ORed, identities
  repointed, games / chat / complaints transferred, friends and blocks
  de-duplicated (friendships keep the strongest status accepted>pending>declined),
  pending invitations/codes dropped, and the secondary kept as an **audit
  tombstone** (`accounts.merged_into`/`merged_at`) so a shared **finished** game's
  no-cascade foreign keys stay valid — its seat there is left untouched. A merge is
  **refused** only when the two share an **active** game. The current account is the
  primary, **except** when the initiator is a **guest** and the linked identity
  already has a **durable** owner: then the durable account wins, the guest's active
  games move into it, the guest is retired, and a **fresh session** is minted for the
  durable account (the client switches to it). The secondary's sessions are revoked
  (§3). High blast-radius; an isolated, well-tested stage.

## 5. Game engine integration (`scrabble-solver`)

`backend` embeds the solver library in-process behind `internal/engine`, the
only package that imports `scrabble-solver` (see [`CLAUDE.md`](../CLAUDE.md) for
the solver's public API and constraints). The engine is a self-contained rules
library — no persistence, transport or scheduling; the game domain drives it.
Key points:

- Variants at launch: **English Scrabble**, **Russian Scrabble**, **Эрудит**
  (`engine.Variant`, mapping to `rules.English()` / `RussianScrabble()` /
  `Erudit()`). Эрудит's specifics (non-doubling centre, `ё` with no tiles, 3
  blanks, a 15-point bonus) live entirely in the solver ruleset, so the engine
  treats every variant uniformly.
- **Dictionaries** are committed DAWGs loaded with `dawg.Load` from the
  directory `BACKEND_DICT_DIR`; `backend` loads the `engine.Registry` at startup
  as a hard dependency (like migrations), so a missing dictionary fails the boot.
  The registry holds dictionaries in memory addressed by `(variant,
  dict_version)`, tracking the latest version per variant, and answers the
  word-check tool through `Registry.Lookup`.
- **Dictionary versioning — pin per game.** A game records the `dict_version` it
  started on and finishes on that version; new games use the latest. Multiple
  versions may be resident at once. The boot version loads from the flat
  `BACKEND_DICT_DIR`; the admin console **hot-reloads** a new version from a
  per-version subdirectory `BACKEND_DICT_DIR/<version>/` through
  `Registry.LoadAvailable` (only the variants whose DAWG is present there), and a
  restart re-loads every resident version via `engine.OpenWithVersions` (the flat
  boot version plus each subdirectory). In-flight games keep their pinned version;
  new games use the latest. (A future split of the solver into engine + dictionary
  generator with versioned artifacts is recorded in [`../PLAN.md`](../PLAN.md)
  TODO-2.)
- Move generation/validation/scoring use `Solver.GenerateMoves` (ranked),
  `Solver.ValidatePlay` and `Solver.ScorePlay`; board mutation uses
  `scrabble.Apply`. The engine adds its own deterministic, seeded tile **bag**
  that can return tiles (an exchange needs this; the solver's self-play bag
  cannot).
- **`engine.Game`** is the in-memory match state and the pure rules engine: it
  deals racks, applies legal plays / passes / exchanges / resignations, refills
  from the bag, keeps the scores and whose turn it is, and **detects the end of
  the game** — empty bag with an empty rack, or six consecutive scoreless turns,
  applying the end-game rack-value adjustment, or a resignation. On a
  **resignation the resigner keeps their accumulated score (no rack adjustment)
  and never wins**: the win goes to the highest score among the remaining seats,
  unconditionally the other player in a two-player game. The engine exposes a
  decoded, solver-free API (`SubmitPlay`/`SubmitExchange`/`EvaluatePlay`/
  `HintView`/`Hand`) so `internal/game` drives it without importing the solver.
- The **game domain** (`internal/game`) owns everything the engine does not —
  persistence, turn scheduling, the configurable turn timeout / auto-resign, the
  hint budget, word-check complaints, history and GCG — and is the engine's only
  consumer. Timeout auto-resign reuses `engine.Resign`, recording the move as a
  timeout, so it inherits the resignation win/loss.
- History is dictionary-independent (§9.1): the engine emits decoded
  `MoveRecord`s and reconstructs the board from them with `engine.ReplayBoard`
  (alphabet only, no dictionary).

## 6. Game rules

- **Word legality: validate-at-submit.** An illegal play is rejected by
  `Solver.ValidatePlay`; there is no challenge phase.
- **End of game**: the bag is empty **and** a player empties their rack, **or**
  **6 consecutive scoreless turns** (passes/exchanges), **or** a resignation, or
  a missed turn. The **per-game turn timeout** is chosen at creation
  (5/10/15/30 min, 1/2/3/6/12/24 h; default 24 h); a turn not made within it
  becomes an automatic resignation, applied by a background sweeper. The sweeper
  honours each player's **away window** — a daily local-time sleep interval on the
  account (default 00:00–07:00, midnight-cross aware) — so a player is never
  timed out while asleep.
- **Players**: auto-match is always 2 players; friend games are 2–4 players.
  `backend` owns turn order and the bag for any player count. A resignation or
  timeout in a two-player game ends it with the other player winning. In a game
  with **three or more seats** a resignation or timeout **drops that seat and the
  rest play on** — the engine skips the resigned seat in the turn rotation and
  excludes it from the win, finishing the game (the sole survivor wins) only once
  one active seat remains, or by the ordinary end conditions among the active
  seats. A per-game **drop-out tile disposition**, chosen at creation
  (`dropout_tiles`: `remove` from play — the default — or `return` to the bag),
  governs the leaver's rack, which is **never revealed** to the remaining players;
  it is recorded for deterministic journal replay. (Two-player games end on the
  first drop-out, so the disposition does not affect them.)
- **Hint**: governed by two per-game settings — whether hints are allowed and the
  starting per-player allowance — plus a per-account hint **wallet**
  (`hint_balance`, spent after the allowance; top-ups are a later feature). A hint
  reveals the top-1 ranked move (`GenerateMoves[0]`). The lobby/tournament caller
  picks the per-game defaults (e.g. one in casual random games, none in
  tournaments). The client **lays the hinted tiles onto the board** as a pending
  placement and leaves the commit to the player. When the rack has no legal move the
  service spends **nothing** and returns `ErrNoHintAvailable` — surfaced as the distinct
  result code `no_hint_available` (separate from `hint_unavailable`) so the UI can say
  "no options" rather than "no hints left".
- **Word-check tool**: unlimited dictionary lookups against the game's pinned
  dictionary; each result offers a **complaint** (complainant, game, variant,
  dict_version, word, the disputed result, an optional note) that lands in the admin
  review queue. An operator resolves it (`open → resolved`) with a **disposition** —
  reject, accept-add or accept-remove; the accepted ones form a derived
  **pending-changes** list that feeds the offline dictionary rebuild and is marked
  applied once the rebuilt version is hot-reloaded (§5, §12).

## 7. Robot opponent

Substitutes for a human in 2-player auto-match when the pool yields no human
within 10 seconds (§8). It lives in `internal/robot` and plays as an ordinary
seated account through the game service, so only `internal/engine` imports the
solver. It is designed to be indistinguishable from a person.

The robot keeps **no per-game state**: every choice is derived deterministically
from the game's bag `seed` (a restart-stable FNV-1a mix), so a background driver
(`robot.Service.Run`, mirroring the turn-timeout sweeper) recomputes the same
behaviour on every scan and after a restart — the same philosophy as journal
replay. A pool of durable accounts — each a `kind='robot'` identity (§4),
provisioned at startup with chat and friend requests blocked — backs the
human-like name pool; those two profile toggles are all the friend/DM blocking
requires (there is no DM surface; chat is per-game).

- **Balance**: at game start it decides once whether to play to win, with
  `P(play-to-win) ≈ 0.40` (so the human wins ≈ 60%), derived from the seed.
  Adaptive difficulty is post-MVP.
- **Margin targeting**: each turn it picks from the ranked candidates
  (`engine.Candidates`) the move whose resulting lead (playing to win) or deficit
  (playing to lose) is closest to a small band (**1–30 points**), rather than
  always the maximum; with no legal play it exchanges a full rack when the bag can
  refill it, else passes.
- **Timing**: per-move delay sampled from a right-skewed distribution (short
  delays frequent, median ≈ 10 min), clamped to **[2, 90] minutes**; it
  **sleeps 00:00–07:00** anchored to the **opponent's** profile timezone with a
  per-game drift of **±3 h** (fallback UTC), so its night overlaps the human's
  rather than running anti-phase; on a daytime nudge it replies within
  **2–10 minutes**; it proactively nudges the human after **12 hours** idle
  (subject to the once-per-hour chat limit).
- **Observability**: robot accounts accrue ordinary statistics (§9) — the
  authoritative balance metric (target ≈ 40% robot wins) — and a
  `robot_games_finished_total` OTel counter plus a per-finish log give a live view.

## 8. Lobby & social

- **Matchmaking**: an **in-memory** FIFO pool keyed by `variant` (the variant
  fixes the board language), pairing the next two humans into a two-player
  auto-match with the seat order randomised for first-move fairness. The pool is
  lost on restart (players re-queue) and is anonymous, so it does not consult
  blocks. After **10 s** with no human a background reaper substitutes a pooled
  robot (§7) and starts the game. On a pairing or substitution the matchmaker
  emits a **match-found** notification (§10), delivered over the live stream;
  `Poll` remains as a fallback for a client that is not currently streaming.
- **Friends** (Stage 8): two add paths over one `friendships` table. A **one-time
  code** the to-be-added player issues (a `friend_codes` row: 6-digit numeric,
  SHA-256-hashed, **12 h** TTL, one live code per issuer, single-use, redeem
  rate-limited) is redeemed by the other player to become friends immediately.
  Alternatively a **request → accept** is sent to someone you **share a game with**
  (active or finished); the recipient may accept, ignore (the pending row lazily
  expires after **30 days** and may be re-sent), or **decline** — a decline is
  remembered (`status='declined'`) and blocks further requests from that sender,
  unless they hand them a code, which overrides it. The requester's own cancel still
  deletes the row; blocking someone severs an existing friendship. (Discovery by
  friend list or platform deep-link arrives with Stage 9 / TODO-5.)
- **Block**: two independent **global** account toggles (`block_chat`,
  `block_friend_requests`) **plus** a **per-user block list**. A per-user block is
  applied mutually: it hides the pair's chat from each other and refuses friend
  requests and game invitations between them.
- **Friend games**: formed by **invitation → accept** (an `game_invitations`
  record with one row per invitee). The 2–4 player game starts once **every**
  invitee accepts; any decline cancels the invitation, and a pending invitation
  expires after 7 days (enforced lazily on access).
- **Chat**: per-game, persisted (kept with the game's archive), **≤ 60 runes**,
  and **validated on input** — links, email addresses and phone numbers (including
  lightly obfuscated forms) are rejected, since the chat is for quick reactions,
  not contact exchange. Each message stores the sender's IP (forwarded by the
  gateway in Stage 6) for moderation. A sender who has disabled chat cannot post,
  and messages from a blocked sender are hidden from the viewer.
- **Nudge**: folded into the chat as a `nudge` message kind. The player awaiting
  the opponent may nudge **once per hour per game**; it is not allowed on one's own
  turn. The platform-native delivery is wired with the gateway / platform
  side-service (Stage 6 / 8).
- **Profile**: `preferred_language` (en/ru, edited in Settings), display name, email
  (confirm-code binding, see §4), **timezone**, the daily **away window** and the
  block toggles — all editable through `account.UpdateProfile`, which validates them
  (Stage 8): a display name is Unicode letters joined by single ` `/`.`/`_`
  separators (no leading/trailing/adjacent separators, ≤ 32 runes); the timezone is a
  fixed `±HH:MM` **UTC offset** (or a legacy IANA name) resolved by `account.ResolveZone`
  for the sweeper and the robot's sleep (a fixed offset trades DST for a simple
  picker); the away window is at most **12 h** (midnight-wrap aware). Linked platform
  accounts and merge are Stage 11.

## 9. Persistence

- Single Postgres database, schema `backend`; `backend` is the only writer. The
  "pgx pool" is a `database/sql` handle backed by the pgx stdlib driver and
  instrumented with otelsql; type-safe queries use **go-jet** (code generated
  into `internal/postgres/jet` and committed, regenerated by `cmd/jetgen`).
  Migrations are embedded SQL applied with `pressly/goose/v3` at startup. Primary
  keys are application-generated **UUIDv7**.
- Tables: `accounts` (durable internal accounts; Stage 3 added the away-window
  columns `away_start`/`away_end` and the hint wallet `hint_balance`; Stage 6's
  migration `00005` added the `is_guest` flag for ephemeral guest rows; Stage 9's
  migration `00007` added the `notifications_in_app_only` out-of-app push toggle;
  Stage 11's migration `00009` added the `paid_account` service flag and the
  merge-tombstone columns `merged_into`/`merged_at`),
  `identities` (platform/email/robot identities, unique `(kind, external_id)`;
  Stage 5's migration `00004` admits the `robot` kind),
  `sessions` (revoke-only opaque-token hashes), the Stage 3 game tables
  `games` (Stage 4 added the `dropout_tiles` disposition column), `game_players`,
  `game_moves` (the move journal), `complaints` and `account_stats`, and the
  Stage 4 social/lobby tables `friendships` (the request/accept graph), `blocks`
  (per-user blocks), `chat_messages` (per-game chat and nudges), `email_confirmations`
  (pending confirm-codes) and `game_invitations` / `game_invitation_invitees`
  (friend-game invitations). Stage 8's migration `00006` widened the `friendships`
  status to admit `declined` and added `friend_codes` (one-time add-a-friend codes).
  The matchmaking pool is **in-memory** and persists nothing.
- **Active games are event-sourced.** A game is a `games` row (pinned
  `variant`/`dict_version`, bag `seed`, the per-game settings, and a denormalised
  turn cursor) plus an append-only, decoded move journal (`game_moves`); the live
  position is an `engine.Game` held in an in-memory cache (≈24 h idle TTL) and
  rebuilt by replaying the journal on a miss, which the seeded bag makes exact.
  Each game is serialised by a per-game lock; a persistence failure evicts the
  live game so the next access rebuilds from the journal. `game_players` records
  each seat's account, running score, hints used and winner flag.
- **Statistics** (`account_stats`, recomputed on each finish for durable
  non-guest accounts only — the finish-time recompute skips any `is_guest`
  seat): wins, losses, **draws**, max points in a game, and
  max points for a single **move** (which already folds in every word the move
  formed plus the all-tiles bonus). A tie increments draws only; a resignation or
  timeout is a loss for the acting player.

### 9.1 History invariant (must hold forever)

Archived games must replay **independently of any dictionary and of the
solver's internal encoding** — at least visually. Therefore the move journal
persists only **decoded concrete values**: action kind (play / pass / exchange /
resign / timeout), acting player, per-move score and running total, timestamp,
and — in a per-move JSON payload — the acting player's rack before the move (with
`?` for a blank), and for a play its direction, main-word anchor, placed tiles
(letter as text, coordinate, blank flag) and the words formed; for an exchange,
the swapped tiles. This is exactly what is needed both to **replay the game
through the engine** (a cache miss) and to render history or emit GCG **without a
dictionary**: the board for visual replay is reconstructed by applying placements
onto an empty grid, since moves were validated at play time and scores are
stored. `variant` and `dict_version` are kept as **metadata only** (audit,
complaint review), never as a replay dependency. **GCG export** is derived from
the same rows and is likewise self-contained — we ship our own writer (the solver
exposes none): the standard Poslfit dialect (UTF-8, `#player`/`#lexicon`
pragmas, `8G`/`H8` coordinates, lower-case blanks, `.` pass-throughs, `-TILES`
exchanges), plus `#note` lines for resignations and timeouts, which the standard
does not cover. **GCG export is offered only on a finished game** (`game.ErrGameActive`
otherwise, Stage 8), so an in-progress journal is never leaked mid-play; the client
shares the `.gcg` file via the Web Share API where available, else downloads it.

The Stage 13 alphabet-on-the-wire change does **not** touch this invariant: the live edge
exchanges alphabet indices, but the persisted journal (and everything derived from it —
replay, history, GCG) keeps the decoded concrete letters described above, so an archived
game still replays with the variant's `rules.Alphabet` alone, independent of any dictionary.

## 10. Notifications

Two channels: the **in-app live stream** (delivered from Stage 6) and
**platform-native push** (out-of-app, via the platform side-service — Stage 9).
The backend emits notification intents through an in-process hub
(`internal/notify`, a `Publisher` seam installed on the game, social and lobby
services); a single backend→gateway **gRPC server-stream** (`Push.Subscribe`,
`pkg/proto/push/v1`) carries every event, and the gateway fans them out by
`user_id` to each client's Connect `Subscribe` stream while the app is open. The
catalog is **your-turn** and **opponent-moved** (emitted from the game commit, so
robot-driver and timeout-sweeper moves emit too), **chat-message** and **nudge**
(from the social service), **match-found** (from the matchmaker, §8), and **notify**
(Stage 8 — a lightweight "re-poll" signal carrying a sub-kind: friend-request,
friend-added, invitation or game-started; emitted on a friend-request and invitation
create and on an invitation's game start). Event payloads are FlatBuffers-encoded by
the backend and forwarded verbatim. A client that is not currently streaming falls
back to the matchmaker's `Poll` for match-found and, for the lobby **notification
badge** (incoming friend requests + open invitations), the client polls on lobby
open and on focus as well as re-polling on the `notify` event — covering a push
missed while the app was hidden. **Out-of-app platform push** (Stage 9) is a fallback
the **gateway** routes from the same firehose: for an event whose recipient has **no
live in-app stream** it resolves the backend `/internal/push-target` (their Telegram
`external_id`, language, and the `notifications_in_app_only` flag) and asks the
**Telegram connector** to deliver a localized message with a Mini App deep-link
button — only when the recipient has a Telegram identity and has not confined
notifications to the app, so the two channels never duplicate. The out-of-app set is
your-turn, nudge, match-found and the invitation / friend-request notify sub-kinds;
the connector renders the message and skips the rest. Session-revocation events and
cursor-based stream resume stay deferred (single-instance MVP).

A separate **announcements channel** feeds the client's one-line banner (UI_DESIGN.md).
It is a client-side **mock** rotation today; a server-driven source (operational notices,
promotions) is future work and would deliver short markdown messages (text + links).

## 11. Observability

- Structured logging with `go.uber.org/zap` (JSON). OpenTelemetry tracer and
  meter providers are wired in **all three services** (backend, gateway, the
  Telegram connector) through a shared `pkg/telemetry` bootstrap, env-gated per
  service by `{BACKEND,GATEWAY,TELEGRAM}_OTEL_{TRACES,METRICS}_EXPORTER` with a
  default of `none` (so no collector is required locally or in CI). `stdout` is
  available for debugging; **`otlp`** (gRPC, endpoint from the standard
  `OTEL_EXPORTER_OTLP_*` environment) exports to a collector. The Postgres pool is
  instrumented with otelsql and `otelgrpc` traces the backend↔gateway push stream
  and the gateway↔connector calls. The OTLP collector and Grafana dashboards are
  stood up with the deploy (Stage 14).
- Per-request server-side timing via gin middleware from day one (the access log
  carries method, route, status, latency and the active trace id). A
  client-measured RTT piggybacked on the next request is a later enhancement.
- Domain/operational metrics (Stage 12), recorded through the meter and invisible
  until an exporter is configured: histograms `game_replay_duration` (journal
  rebuild on a cache miss) and `game_move_validate_duration`; counters
  `games_started_total`, `games_abandoned_total` (a turn-timeout seat drop),
  `chat_messages_total` (`kind` = message/nudge) and `robot_games_finished_total`;
  an observable gauge `game_cache_active`; the gateway `edge_request_duration`
  (the UI-perceived roundtrip, by `message_type`/`result`); and Go runtime/heap
  metrics. Game-scoped metrics carry a `variant` attribute
  (english/russian_scrabble/erudit).
- Unauthenticated `GET /healthz` (liveness) and `GET /readyz` (readiness — the
  database answers a bounded ping and the session cache is warmed).
- The backend serves a **second listener** — a gRPC server
  (`BACKEND_GRPC_ADDR`, default `:9090`) for the live-event push stream to the
  gateway — alongside the HTTP listener; both start together and stop on signal.

## 12. Security boundaries

| Concern | Enforced by |
| --- | --- |
| Public rate limiting / anti-abuse | gateway |
| Telegram initData validation (bot-token HMAC) | the Telegram connector; the gateway delegates it over gRPC, so the bot token lives only in the connector |
| Session minting; email-code / guest validation | gateway (with backend) |
| Session → `user_id` resolution, `X-User-ID` injection | gateway |
| Authorisation, ownership, state transitions | backend (`X-User-ID` is the sole identity input) |
| Admin authentication | gateway validates HTTP Basic Auth (`GATEWAY_ADMIN_*`) on the public `/_gm/*` path and reverse-proxies it **verbatim** to the backend's server-rendered admin console; the backend trusts the gateway (no admin principal) and guards its state-changing POSTs with a **same-origin** check — the console's CSRF defence. No operator identity is tracked |
| backend ↔ gateway ↔ connector trust | the network (only gateway may reach backend; the connector serves unauthenticated gRPC on the internal segment) |

This is an explicit, accepted MVP risk: compromise of the gateway↔backend
network segment defeats backend authentication. Mitigated by network isolation;
mutual auth is a future hardening step.

**Short numeric codes** (email confirm-codes and Stage 8 friend codes) are stored
only as SHA-256 hashes and are short-lived and single-use. The unauthenticated
email path carries a tight per-IP sub-limit (5 / 10 min); the **friend-code redeem**
is authenticated, so it rides the per-user limit (120 / min) and is further bounded
by the code's 12 h TTL, single use, and **one live code per issuer** (which caps the
valid-code population). Brute-forcing a 6-digit friend code within these limits is an
accepted MVP risk with low blast radius (an unwanted friendship is removable/blockable);
a dedicated redeem sub-limit or a longer code is the hardening step if abuse appears.

## 13. Deployment (informational)

Single public origin, path-routed: a mini-landing at the root, the **Telegram Mini
App under `/telegram/`** (the gateway serves the static UI build, wired in Stage 14;
outside Telegram that path redirects to the root), the gateway public surface and the **admin console
at `/_gm`** (backend-rendered, Basic-Auth at the gateway) share one host that
terminates TLS. The **Telegram connector** runs as a separate
container with **no public ingress** — it long-polls Telegram and egresses through a
VPN sidecar, answering only internal gRPC. MVP runs one `gateway`, one `backend`, one
Postgres, plus the connector. The connector's Docker/compose ships now
(`platform/telegram/deploy`, mirroring `../15-puzzle`); the gateway's static UI serving
and the full multi-service deploy land in Stage 14.

## 14. CI & branches

- Trunk is **`master`**; feature work happens on `feature/*` branches merged via
  PR with a green CI gate (from Stage 1 onward — the genesis commit necessarily
  lands on `master`).
- `.gitea/workflows/` holds the CI. `go-unit.yaml` runs gofmt/vet/build/unit-test
  on Go changes; `integration.yaml` runs the Postgres-backed tests behind the
  `integration` build tag (testcontainers `postgres:17-alpine`, Ryuk disabled,
  serial). Further workflows (ui-test, deploy) are added with the components they
  cover.
- Since Stage 2 both Go workflows clone the public `scrabble-solver` sibling
  (master HEAD, no credentials) into `../scrabble-solver` before building, so the
  `go.work` `replace` resolves; the engine tests read the committed DAWGs from
  that checkout via `BACKEND_DICT_DIR`.
- After any push, the run is watched to green before a stage is declared done
  (`python3 ~/.claude/bin/gitea-ci-watch.py`).