feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

gateway/cmd/gateway/main.go

@@ -126,7 +126,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		logger.Info("admin console disabled (set GATEWAY_ADMIN_USER and GATEWAY_ADMIN_PASSWORD)")
 	}
 
-	registry := transcode.NewRegistry(backend, validator, cfg.DefaultSupportedLanguages...)
+	registry := transcode.NewRegistry(backend, validator)
 	edge := connectsrv.NewServer(connectsrv.Deps{
 		Registry:     registry,
 		Sessions:     sessions,
@@ -258,7 +258,7 @@ func runPushPump(ctx context.Context, backend *backendclient.Client, hub *push.H
 			// deliver the event over the platform push channel. Done in a goroutine
 			// so a slow connector never stalls the in-app firehose.
 			if conn != nil && connector.OutOfAppKind(ev.GetKind()) && !hub.HasSubscribers(ev.GetUserId()) {
-				go deliverOutOfApp(ctx, backend, conn, ev.GetUserId(), ev.GetKind(), ev.GetPayload(), ev.GetLanguage(), logger)
+				go deliverOutOfApp(ctx, backend, conn, ev.GetUserId(), ev.GetKind(), ev.GetPayload(), logger)
 			}
 		}
 		if !sleep(ctx, pushReconnectDelay) {
@@ -271,7 +271,7 @@ func runPushPump(ctx context.Context, backend *backendclient.Client, hub *push.H
 // Telegram identity and have not confined notifications to the app, asks the
 // connector to deliver the event. It is best-effort: every failure is logged and
 // dropped (the in-app stream remains the primary channel).
-func deliverOutOfApp(ctx context.Context, backend *backendclient.Client, conn *connector.Client, userID, kind string, payload []byte, gameLang string, logger *zap.Logger) {
+func deliverOutOfApp(ctx context.Context, backend *backendclient.Client, conn *connector.Client, userID, kind string, payload []byte, logger *zap.Logger) {
 	target, err := backend.PushTarget(ctx, userID)
 	if err != nil {
 		logger.Warn("push target lookup failed", zap.String("user_id", userID), zap.Error(err))
@@ -280,13 +280,8 @@ func deliverOutOfApp(ctx context.Context, backend *backendclient.Client, conn *c
 	if !connector.DeliverToTarget(target.ExternalID, target.NotificationsInAppOnly) {
 		return
 	}
-	// A game event carries its own language, so the push comes from the game's bot rather than
-	// the recipient's last-login bot; other events fall back to the service language.
-	lang := target.Language
-	if gameLang != "" {
-		lang = gameLang
-	}
-	if _, err := conn.Notify(ctx, target.ExternalID, kind, payload, lang); err != nil {
+	// The single bot renders the message in the recipient's interface language.
+	if _, err := conn.Notify(ctx, target.ExternalID, kind, payload, target.Language); err != nil {
 		logger.Warn("out-of-app notify failed", zap.String("kind", kind), zap.Error(err))
 	}
 }