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/service.go

@@ -9,6 +9,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/google/uuid"
@@ -30,12 +31,16 @@ type Service struct {
 	registry *engine.Registry
 	cache    *gameCache
 	locks    *keyedMutex
-	version  string
-	clock    func() time.Time
-	rng      func() int64
-	pub      notify.Publisher
-	metrics  *gameMetrics
-	log      *zap.Logger
+	// verMu guards version, the dictionary version new games pin. It is read on
+	// the game-create path and rewritten when an operator activates a new version
+	// through the admin console, so access is serialised.
+	verMu   sync.RWMutex
+	version string
+	clock   func() time.Time
+	rng     func() int64
+	pub     notify.Publisher
+	metrics *gameMetrics
+	log     *zap.Logger
 }
 
 // NewService constructs a Service. store and accounts wrap the same pool;
@@ -70,6 +75,62 @@ func (svc *Service) SetNotifier(p notify.Publisher) {
 	}
 }
 
+// activeVersion returns the dictionary version new games currently pin.
+func (svc *Service) activeVersion() string {
+	svc.verMu.RLock()
+	defer svc.verMu.RUnlock()
+	return svc.version
+}
+
+// ActiveVersion returns the dictionary version new games currently pin. It backs
+// the admin console's "active version" display.
+func (svc *Service) ActiveVersion() string {
+	return svc.activeVersion()
+}
+
+// InitActiveVersion reconciles the persisted active dictionary version with the
+// registry at startup. If a version was persisted and is resident, it becomes the
+// active version; otherwise the configured seed version is kept and persisted as
+// the initial active version. Call once during wiring, before serving traffic.
+func (svc *Service) InitActiveVersion(ctx context.Context) error {
+	persisted, ok, err := svc.store.GetActiveDictVersion(ctx)
+	if err != nil {
+		return err
+	}
+	if ok && svc.versionResident(persisted) {
+		svc.verMu.Lock()
+		svc.version = persisted
+		svc.verMu.Unlock()
+		return nil
+	}
+	return svc.store.SetActiveDictVersion(ctx, svc.activeVersion())
+}
+
+// SetActiveVersion records version as the active dictionary version, persisting it
+// and updating the in-memory pointer new games read. The caller must have made the
+// version resident in the registry first.
+func (svc *Service) SetActiveVersion(ctx context.Context, version string) error {
+	if err := svc.store.SetActiveDictVersion(ctx, version); err != nil {
+		return err
+	}
+	svc.verMu.Lock()
+	svc.version = version
+	svc.verMu.Unlock()
+	return nil
+}
+
+// versionResident reports whether any variant of version is loaded in the
+// registry, so a persisted active version pointing at a no-longer-present version
+// (e.g. after a volume loss) falls back to the seed.
+func (svc *Service) versionResident(version string) bool {
+	for _, v := range engine.Variants() {
+		if _, err := svc.registry.Solver(v, version); err == nil {
+			return true
+		}
+	}
+	return false
+}
+
 // Create starts and persists a new game seating the given accounts in turn order
 // (seat 0 first), deals the racks, and warms the live-game cache. It validates
 // the player count (2–4), the move clock, the hint allowance and that every seat
@@ -106,9 +167,12 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 	if seed == 0 {
 		seed = svc.rng()
 	}
+	// Capture the active version once so the engine game and the persisted
+	// dict_version agree even if an operator activates a new version concurrently.
+	version := svc.activeVersion()
 	g, err := engine.New(svc.registry, engine.Options{
 		Variant:              params.Variant,
-		Version:              svc.version,
+		Version:              version,
 		Players:              len(params.Seats),
 		Seed:                 seed,
 		DropoutTiles:         params.DropoutTiles,
@@ -128,7 +192,7 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 	ins := gameInsert{
 		id:                   id,
 		variant:              params.Variant.String(),
-		dictVersion:          svc.version,
+		dictVersion:          version,
 		seed:                 seed,
 		players:              len(params.Seats),
 		turnTimeoutSecs:      int(timeout / time.Second),
@@ -180,7 +244,7 @@ func (svc *Service) OpenOrJoin(ctx context.Context, accountID uuid.UUID, params
 	ins := gameInsert{
 		id:                   id,
 		variant:              params.Variant.String(),
-		dictVersion:          svc.version,
+		dictVersion:          svc.activeVersion(),
 		seed:                 seed,
 		players:              2,
 		turnTimeoutSecs:      int(timeout / time.Second),
@@ -759,7 +823,7 @@ func (svc *Service) DictionaryChanges(ctx context.Context) ([]DictionaryChange,
 }
 
 // MarkChangesApplied records that every pending accepted change for variant has
-// been folded into the dictionary version that was just hot-reloaded, removing
+// been folded into the dictionary version that was just installed, removing
 // them from DictionaryChanges. It returns the number of changes marked.
 func (svc *Service) MarkChangesApplied(ctx context.Context, variant engine.Variant, version string) (int64, error) {
 	return svc.store.MarkChangesApplied(ctx, variant.String(), version)