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.

backend/internal/game/store.go

@@ -767,6 +767,41 @@ func (s *Store) MarkChangesApplied(ctx context.Context, variant, version string)
 	return n, nil
 }
 
+// GetActiveDictVersion returns the persisted active dictionary version and true,
+// or ("", false, nil) when none has been recorded yet (a fresh database). It reads
+// the dictionary_state singleton (docs/ARCHITECTURE.md §5).
+func (s *Store) GetActiveDictVersion(ctx context.Context) (string, bool, error) {
+	stmt := postgres.SELECT(table.DictionaryState.ActiveVersion).
+		FROM(table.DictionaryState).
+		WHERE(table.DictionaryState.ID.EQ(postgres.Bool(true)))
+	var row model.DictionaryState
+	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
+		if errors.Is(err, qrm.ErrNoRows) {
+			return "", false, nil
+		}
+		return "", false, fmt.Errorf("game: get active dict version: %w", err)
+	}
+	return row.ActiveVersion, true, nil
+}
+
+// SetActiveDictVersion records version as the active dictionary version, upserting
+// the dictionary_state singleton so the choice survives a restart.
+func (s *Store) SetActiveDictVersion(ctx context.Context, version string) error {
+	now := time.Now().UTC()
+	stmt := table.DictionaryState.
+		INSERT(table.DictionaryState.ID, table.DictionaryState.ActiveVersion, table.DictionaryState.UpdatedAt).
+		VALUES(true, version, postgres.TimestampzT(now)).
+		ON_CONFLICT(table.DictionaryState.ID).
+		DO_UPDATE(postgres.SET(
+			table.DictionaryState.ActiveVersion.SET(postgres.String(version)),
+			table.DictionaryState.UpdatedAt.SET(postgres.TimestampzT(now)),
+		))
+	if _, err := stmt.ExecContext(ctx, s.db); err != nil {
+		return fmt.Errorf("game: set active dict version: %w", err)
+	}
+	return nil
+}
+
 // CountComplaints returns the number of complaints, optionally restricted to a
 // status, for the admin queue pager and the dashboard counts.
 func (s *Store) CountComplaints(ctx context.Context, status string) (int, error) {