Stage 6: gateway edge (Connect/FlatBuffers over h2c, platform/email/guest auth, sessions, rate-limit, admin passthrough, live push bridge)

New public ingress and the first network edge. Framework + a vertical slice of
operations end-to-end; remaining ops reuse the same transcode pattern in Stage 7.

Contracts (new module scrabble/pkg):
- push.proto (backend->gateway gRPC server-stream) + scrabble.fbs (FlatBuffers
  edge payloads), committed generated Go; buf/flatc Makefiles (dev-time codegen).

Backend:
- REST handlers on the /api/v1 groups: internal session endpoints
  (telegram/guest/email login -> mint, resolve, revoke) and the user slice
  (profile, submit_play, state, lobby enqueue/poll, chat).
- internal/notify in-process Publisher hub + internal/pushgrpc gRPC server
  (BACKEND_GRPC_ADDR) streaming your_turn/opponent_moved/chat/nudge/match_found;
  emission in game.commit, social, matchmaker.
- migration 00005 accounts.is_guest; guests are durable rows excluded from stats;
  ProvisionGuest; email-as-login (RequestLoginCode/LoginWithCode).

Gateway (new module scrabble/gateway):
- Connect Gateway service over h2c (Execute + Subscribe), FlatBuffers<->JSON
  transcode registry, Telegram initData HMAC validator (seam), session cache,
  token-bucket rate limiter (3 classes), push fan-out hub, backend REST + push
  gRPC client, admin Basic-Auth reverse proxy.

go.work: use ./pkg, ./gateway + replace scrabble/pkg. CI: gateway/**, pkg/**
path filters; unit build/vet/test span all three modules. Docs (PLAN,
ARCHITECTURE, FUNCTIONAL+ru, TESTING, READMEs) updated; gateway/pkg unit tests +
guest/email-login integration tests.

pkg/fbs/scrabble.fbs

@@ -0,0 +1,205 @@
+// FlatBuffers payloads for the client <-> gateway edge transport (Stage 6).
+//
+// Every request and response that rides inside the Connect envelope
+// (gateway/proto/edge) `payload` field, and every push Event payload, is one of
+// these tables. They are the binary wire contract shared with the UI, which
+// generates TypeScript from this same schema (Stage 7). A single namespace keeps
+// nested tables (GameView inside MoveResult / MatchResult) free of
+// cross-namespace imports. Keep this schema and the backend JSON DTOs in lockstep
+// — the gateway transcodes one to the other.
+//
+// Generate Go with `make fbs` (flatc, version-pinned). The committed output lives
+// in fbs/scrabblefb/.
+namespace scrabblefb;
+
+// --- shared building blocks ---
+
+// TileRecord is one placed (or to-place) tile: its board coordinate, the concrete
+// letter ("?" when read from a hand for a blank) and whether it came from a blank.
+table TileRecord {
+  row:int;
+  col:int;
+  letter:string;
+  blank:bool;
+}
+
+// SeatView is one seat's public standing in a game.
+table SeatView {
+  seat:int;
+  account_id:string;
+  score:int;
+  hints_used:int;
+  is_winner:bool;
+}
+
+// GameView is the shared (non-private) game summary.
+table GameView {
+  id:string;
+  variant:string;
+  dict_version:string;
+  status:string;
+  players:int;
+  to_move:int;
+  turn_timeout_secs:int;
+  move_count:int;
+  end_reason:string;
+  seats:[SeatView];
+}
+
+// MoveRecord is one decoded move (a committed play, or a hint preview).
+table MoveRecord {
+  player:int;
+  action:string;
+  dir:string;
+  main_row:int;
+  main_col:int;
+  tiles:[TileRecord];
+  words:[string];
+  count:int;
+  score:int;
+  total:int;
+}
+
+// --- auth (unauthenticated) ---
+
+// TelegramLoginRequest carries the platform launch data; the gateway validates
+// its HMAC before forwarding the extracted identity to the backend.
+table TelegramLoginRequest {
+  init_data:string;
+}
+
+// GuestLoginRequest bootstraps an ephemeral guest session. locale is an optional
+// preferred-language hint.
+table GuestLoginRequest {
+  locale:string;
+}
+
+// EmailRequestRequest asks the backend to send a login confirm-code to email.
+table EmailRequestRequest {
+  email:string;
+}
+
+// EmailLoginRequest logs in (or provisions) the account owning email, verifying
+// the confirm-code.
+table EmailLoginRequest {
+  email:string;
+  code:string;
+}
+
+// Session is the minted credential returned by every auth operation.
+table Session {
+  token:string;
+  user_id:string;
+  is_guest:bool;
+  display_name:string;
+}
+
+// Ack is a simple success acknowledgement (e.g. an email-code request).
+table Ack {
+  ok:bool;
+}
+
+// --- profile (authenticated) ---
+
+// Profile is the authenticated account's own profile view.
+table Profile {
+  user_id:string;
+  display_name:string;
+  preferred_language:string;
+  time_zone:string;
+  hint_balance:int;
+  block_chat:bool;
+  block_friend_requests:bool;
+  is_guest:bool;
+}
+
+// --- game (authenticated) ---
+
+// SubmitPlayRequest places tiles in a direction on the player's turn.
+table SubmitPlayRequest {
+  game_id:string;
+  dir:string;
+  tiles:[TileRecord];
+}
+
+// MoveResult is the outcome of a committed move: the move and the post-move game.
+table MoveResult {
+  move:MoveRecord;
+  game:GameView;
+}
+
+// StateRequest asks for the requesting player's view of a game.
+table StateRequest {
+  game_id:string;
+}
+
+// StateView is a player's view of a game: the shared summary plus their private
+// rack, the bag size and their remaining hint budget.
+table StateView {
+  game:GameView;
+  seat:int;
+  rack:[string];
+  bag_len:int;
+  hints_remaining:int;
+}
+
+// --- lobby (authenticated) ---
+
+// EnqueueRequest joins the per-variant auto-match pool.
+table EnqueueRequest {
+  variant:string;
+}
+
+// MatchResult reports whether the caller has been paired into a game yet.
+table MatchResult {
+  matched:bool;
+  game:GameView;
+}
+
+// --- chat (authenticated) ---
+
+// ChatPostRequest posts a per-game chat message.
+table ChatPostRequest {
+  game_id:string;
+  body:string;
+}
+
+// ChatMessage is one stored chat message or nudge.
+table ChatMessage {
+  id:string;
+  game_id:string;
+  sender_id:string;
+  kind:string;
+  body:string;
+  created_at_unix:long;
+}
+
+// --- push event payloads ---
+
+// YourTurnEvent signals that it is now the recipient's turn.
+table YourTurnEvent {
+  game_id:string;
+  deadline_unix:long;
+}
+
+// OpponentMovedEvent summarises a move another seat just committed; the client
+// refetches the full state.
+table OpponentMovedEvent {
+  game_id:string;
+  seat:int;
+  action:string;
+  score:int;
+  total:int;
+}
+
+// NudgeEvent signals that a player nudged the recipient.
+table NudgeEvent {
+  game_id:string;
+  from_user_id:string;
+}
+
+// MatchFoundEvent signals that an auto-match pairing (or robot substitution)
+// started a game the recipient is seated in.
+table MatchFoundEvent {
+  game_id:string;
+}