feat(admin): online dictionary update — upload archive, preview word diff, install & activate

Replace the dictionary hot-reload with an online update flow in the GM console
(/_gm/dictionary): the operator uploads scrabble-dawg-vX.Y.Z.tar.gz, previews the
per-variant words added/removed against the active dictionary, and confirms to
install + activate it. Versions are immutable; in-progress games keep their pinned
version while new games use the new one.

- engine: DiffWords (enumerate both DAWGs, decode only the differences), OpenFinder,
  Registry.Finder, DictFiles; OpenWithVersions skips the .staging area.
- dictadmin: hardened release-archive validation + extraction (path-traversal,
  symlink, oversize, entry-count rejection) and staging -> install (atomic rename).
- game: active dictionary version persisted in the dictionary_state singleton
  (single source of truth, restored on boot), concurrency-safe accessor.
- storage: BACKEND_DICT_DIR is a named volume seeded from the image (nonroot-owned),
  so uploaded versions persist across redeploys; the build's DICT_VERSION labels the
  seed and equals the resident tag (BACKEND_DICT_VERSION).
- docs: ARCHITECTURE §5, FUNCTIONAL (+ru), backend README, TESTING, PRERELEASE.

Tests: engine + dictadmin unit; integration upload->preview->install->activate->
restart->pin->immutability->CSRF.

deploy/docker-compose.yml

@@ -90,6 +90,15 @@ services:
       # GOMAXPROCS matches the CPU limit below so the Go scheduler aligns with the
       # cgroup quota (the runtime otherwise sees all of the host's cores).
       GOMAXPROCS: "2"
+    # The dictionary lives on a named volume seeded from the image on first boot
+    # (the image's /opt/dawg is owned by the nonroot UID, which the fresh volume
+    # inherits). The admin console writes new version subdirectories here, and the
+    # volume preserves them — and the versions in-progress games pin — across
+    # redeploys. Once seeded the volume is not re-seeded, so bumping DICT_VERSION on a
+    # later deploy has no effect on an existing contour; update via the console
+    # instead (docs/ARCHITECTURE.md §5).
+    volumes:
+      - dawg-data:/opt/dawg
     # No container healthcheck: the distroless image has no shell/wget. Readiness
     # is covered by the CI post-deploy probe (GET / through caddy).
     # R7 starting limits (generous over the R2 ~1-core / <=100 MiB peak); tightened to
@@ -366,6 +375,7 @@ networks:
 
 volumes:
   postgres-data:
+  dawg-data:
   caddy-data:
   prometheus-data:
   tempo-data: