feat(engine,deploy): seed-drift guard + track current dictionary release

The dictionary release moved to v1.2.1 while DICT_VERSION stayed pinned at
v1.0.0 in CI and the image/compose seed defaults. Two problems:

1. CI validated against a stale dictionary.
2. The contour seed could be bumped on a live volume, which silently relabels
   the already-seeded bytes — voiding games pinned to the prior label and
   serving the wrong dictionary for new ones. The flat DAWGs carry no embedded
   version, so this drift was undetectable.

Changes:

- Seed-drift guard: OpenWithVersions records the flat dir's version in a
  .seed_version marker on first boot and refuses to start when a later
  BACKEND_DICT_VERSION disagrees. DICT_VERSION is now the seed for a *fresh*
  volume only; a live contour migrates through the admin console (old versions
  stay resident, in-progress games keep replaying).
- Track the current release: CI's DICT_VERSION centralised to one workflow-level
  env (v1.2.1); image/compose/.env seed defaults bumped to v1.2.1. The deploy
  job keeps reading the per-contour vars.TEST_DICT_VERSION.
- Docs: ARCHITECTURE §5 (decision record), backend/deploy READMEs, PRERELEASE
  tracker (DV row).

Verified locally against the v1.2.1 artifact: gofmt, build, vet, unit and
integration (-tags=integration) all green.

PRERELEASE.md

@@ -38,6 +38,7 @@ the edge before prod. Each phase maps back to the owner's raw pre-release TODO l
 | BX | Asymmetric per-user block + in-game controls: a block now silently suppresses everything **from** the blocked user (chat, nudge, friend requests, invitations are kept but never delivered/surfaced, born-read) while they notice nothing, **without** deleting the friendship (unblock restores it); auto-match excludes a block-related pair (either direction); in-game opponent card gains a ✖️ **block** control (mirroring 🤝, red "Block?" confirm, mutual-hide, struck name + hidden chat composer when blocked); optimistic apply + `user_blocked`/`user_unblocked` event confirm + rollback; admin user card gains **blocks / blocked-by / friends** cross-linked lists. Blocking a disguised-robot opponent is recorded per-game in a separate **`robot_blocks`** table (migration `00011`), keyed on game+seat with the seen name — never the shared robot account — so the matchmaker keeps giving robots; it shows in the blocked list and re-marks the in-game card | owner ad-hoc | **done** |
 | FM | First-move tile draw (official rules): each seated player draws a tile, the one closest to "A" leads (a blank beats every letter), ties re-drawing until a single leader; **honest per-draw `crypto/rand` entropy**, not the bag seed, so the **record** (`game_setup_draws`, migration `00013`) — not a seed — is the only account of the outcome, kept for future **tournaments** (designed as a discrete per-tile "player N draws" step). Friend/AI draws at create; **auto-match draws at *open*** against a synthetic `uuid.Nil` opponent whose draw rows are back-filled on join, so the opener's seat is fixed up front and the existing open-game pre-move is preserved (no reseating, no play-gating). Admin `/_gm/games/:id` gains the recorded draw list + a simple **step-by-step board replay** (`ReplayTimeline`). | owner ad-hoc | **done** |
 | SB | Single Telegram bot + per-user variant preferences: the two per-language bots collapse into **one** (drop `accounts.service_language`, `supported_languages`, the `*_EN`/`*_RU` env vars and game-language push routing — the single bot renders in the recipient's `preferred_language`); New Game variant gating moves to a profile **`variant_preferences`** set (default Erudit only, Erudit-first, server-enforced on the caller's auto-match/vs-AI/invitation-create paths, an invited friend may accept any variant); env vars collapse to unsuffixed `TELEGRAM_BOT_TOKEN`/`TELEGRAM_GAME_CHANNEL_ID`/`VITE_TELEGRAM_LINK`/`VITE_TELEGRAM_GAME_CHANNEL_NAME` and `GATEWAY_DEFAULT_SUPPORTED_LANGUAGES` is removed; wire drops `service_language`/`supported_languages` (Session, ValidateInitDataResponse) + the push `language` routing field and adds `variant_preferences` to Profile/UpdateProfile. | owner ad-hoc | **done** |
+| DV | Dictionary version hygiene: CI + image/compose seed track the current release (`v1.2.1`); a **seed-drift guard** records the flat dir's seed in a `.seed_version` marker and refuses to boot when a bumped build seed would relabel a live volume (silently serving the wrong dictionary + voiding games pinned to the prior label); `DICT_VERSION` is the fresh-volume seed only, a live contour migrates through the admin console | owner ad-hoc | **done** |
 | → | Stage 18 — prod contour deploy | — | see [`PLAN.md`](PLAN.md) |
 
 ## Key findings (these reshaped the raw list — read before starting a phase)