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.

deploy/.env.example

@@ -10,12 +10,13 @@ POSTGRES_USER=scrabble
 POSTGRES_PASSWORD=change-me            # required
 
 # --- Dictionary -------------------------------------------------------------
-# scrabble-dictionary release tag baked into the image as the SEED dictionary
-# (image build-arg; also labels the resident seed version). After first boot the
-# dawg-data volume preserves versions uploaded through the admin console, and the
-# active version lives in the DB — so bumping this on a later deploy does NOT change
-# a running contour; update the dictionary through /_gm/dictionary instead.
-DICT_VERSION=v1.0.0
+# scrabble-dictionary release tag baked into the image as the SEED dictionary for a
+# FRESH volume (image build-arg; also labels the resident seed version). After first
+# boot the dawg-data volume preserves versions uploaded through the admin console and
+# the active version lives in the DB. On a live volume the backend refuses to start if
+# this no longer matches the recorded seed (the seed-drift guard) — change a running
+# contour's dictionary through /_gm/dictionary, not by bumping this (ARCHITECTURE.md §5).
+DICT_VERSION=v1.2.1
 
 # --- Logging ----------------------------------------------------------------
 LOG_LEVEL=info