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.

backend/README.md

@@ -190,7 +190,7 @@ internal/ratewatch/  # gateway rate-limit reports: episode window for the consol
 | `BACKEND_OTEL_TRACES_EXPORTER` | `none` | `none`, `stdout` or `otlp` (gRPC; endpoint from the standard `OTEL_EXPORTER_OTLP_*`). |
 | `BACKEND_OTEL_METRICS_EXPORTER` | `none` | `none`, `stdout` or `otlp`. |
 | `BACKEND_DICT_DIR` | — | **Required.** Directory of committed `.dawg` dictionaries. |
-| `BACKEND_DICT_VERSION` | `v1` | Dictionary version new games pin. |
+| `BACKEND_DICT_VERSION` | `v1` | Seed dictionary version new games pin (the flat dir's label). Recorded in a `.seed_version` marker on first boot; the backend refuses to start if a later value drifts from it — the seed-drift guard (ARCHITECTURE.md §5). |
 | `BACKEND_GAME_TIMEOUT_SWEEP_INTERVAL` | `1m` | How often the turn-timeout sweeper runs. |
 | `BACKEND_GAME_CACHE_TTL` | `24h` | Idle window before a live game is evicted from cache. |
 | `BACKEND_LOBBY_ROBOT_WAIT` | `10s` | Auto-match wait before a robot is substituted for a missing human. |
@@ -212,7 +212,7 @@ internal/ratewatch/  # gateway rate-limit reports: episode window for the consol
 ```sh
 docker run -d --name scrabble-pg -e POSTGRES_PASSWORD=dev -p 5432:5432 postgres:17-alpine
 # DAWGs: extract the dictionary release artifact (or point at a local scrabble-solver/dawg):
-mkdir -p /tmp/dawg && curl -fsSL https://gitea.iliadenisov.ru/developer/scrabble-dictionary/releases/download/v1.0.0/scrabble-dawg-v1.0.0.tar.gz | tar xz -C /tmp/dawg
+mkdir -p /tmp/dawg && curl -fsSL https://gitea.iliadenisov.ru/developer/scrabble-dictionary/releases/download/v1.2.1/scrabble-dawg-v1.2.1.tar.gz | tar xz -C /tmp/dawg
 BACKEND_POSTGRES_DSN='postgres://postgres:dev@localhost:5432/postgres?search_path=backend&sslmode=disable' \
   BACKEND_DICT_DIR=/tmp/dawg \
   GOPRIVATE='gitea.iliadenisov.ru/*' \
@@ -251,7 +251,13 @@ local solver co-development you may add a temporary replace — see `go.work`).
 from the [`scrabble-dictionary`](https://gitea.iliadenisov.ru/developer/scrabble-dictionary)
 repo (one semver per set); the engine loads them by `(variant, dict_version)` from
 `BACKEND_DICT_DIR`. The backend loads them at startup as a hard dependency
-(a missing dictionary aborts the boot).
+(a missing dictionary aborts the boot). The flat directory is the seed version,
+labelled `BACKEND_DICT_VERSION`; uploaded versions live in `<version>/`
+subdirectories the admin console writes and a restart re-loads. Because the DAWGs
+carry no embedded version, the first boot records the seed in a `.seed_version`
+marker and a later boot **refuses to start** if `BACKEND_DICT_VERSION` no longer
+matches it (the seed-drift guard), so a live contour's dictionary is changed through
+the console, never by bumping the build seed (ARCHITECTURE.md §5).
 
 ## Tests