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.

.gitea/workflows/ci.yaml

@@ -26,6 +26,13 @@ on:
   push:
     branches: [development]
 
+# The dictionary release the test suite validates against — the current
+# scrabble-dictionary release. Centralised here so a release bump is one edit; the
+# unit/integration jobs inherit it. The deploy job overrides it per contour with
+# vars.TEST_DICT_VERSION (the seed for a fresh volume), see deploy/README.md.
+env:
+  DICT_VERSION: v1.2.1
+
 jobs:
   # changes detects which areas a PR/push touched, so the test jobs can skip when
   # irrelevant. It defaults to running everything when the diff cannot be computed.
@@ -86,7 +93,6 @@ jobs:
       # The engine consumes the published scrabble-solver module from this Gitea;
       # GOPRIVATE makes go fetch it directly (skipping the public proxy/checksum DB).
       GOPRIVATE: gitea.iliadenisov.ru/*
-      DICT_VERSION: v1.0.0
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -134,7 +140,6 @@ jobs:
       # suite's TestMain terminates its own container, so disable it.
       TESTCONTAINERS_RYUK_DISABLED: "true"
       GOPRIVATE: gitea.iliadenisov.ru/*
-      DICT_VERSION: v1.0.0
     steps:
       - name: Checkout
         uses: actions/checkout@v4