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/internal/transcode/transcode.go

@@ -89,20 +89,14 @@ type TelegramValidator interface {
 // NewRegistry builds the slice's message-type catalog over the backend client.
 // The Telegram auth op is registered only when a validator is supplied (the
 // connector is configured); otherwise auth.telegram is simply unknown.
-// defaultLanguages is the New Game variant gating set placed on the Session for
-// non-platform logins (web / email / guest) and on a switched link session; an
-// empty argument falls back to all languages (matching the config default).
-func NewRegistry(backend *backendclient.Client, tg TelegramValidator, defaultLanguages ...string) *Registry {
-	if len(defaultLanguages) == 0 {
-		defaultLanguages = []string{"en", "ru"}
-	}
+func NewRegistry(backend *backendclient.Client, tg TelegramValidator) *Registry {
 	r := &Registry{ops: make(map[string]Op)}
 	if tg != nil {
 		r.ops[MsgAuthTelegram] = Op{Handler: authTelegramHandler(backend, tg)}
 	}
-	r.ops[MsgAuthGuest] = Op{Handler: authGuestHandler(backend, defaultLanguages)}
+	r.ops[MsgAuthGuest] = Op{Handler: authGuestHandler(backend)}
 	r.ops[MsgAuthEmailReq] = Op{Handler: authEmailRequestHandler(backend), Email: true}
-	r.ops[MsgAuthEmailLogin] = Op{Handler: authEmailLoginHandler(backend, defaultLanguages), Email: true}
+	r.ops[MsgAuthEmailLogin] = Op{Handler: authEmailLoginHandler(backend), Email: true}
 	r.ops[MsgProfileGet] = Op{Handler: profileHandler(backend), Auth: true}
 	r.ops[MsgBlockStatus] = Op{Handler: blockStatusHandler(backend), Auth: true}
 	r.ops[MsgGameSubmitPlay] = Op{Handler: submitPlayHandler(backend), Auth: true}
@@ -130,7 +124,7 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, defaultLan
 	r.ops[MsgFeedbackGet] = Op{Handler: feedbackGetHandler(backend), Auth: true}
 	r.ops[MsgFeedbackUnread] = Op{Handler: feedbackUnreadHandler(backend), Auth: true}
 	registerSocialOps(r, backend)
-	registerLinkOps(r, backend, tg, defaultLanguages)
+	registerLinkOps(r, backend, tg)
 	return r
 }
 
@@ -164,21 +158,21 @@ func authTelegramHandler(backend *backendclient.Client, tg TelegramValidator) Ha
 		if err != nil {
 			return nil, err
 		}
-		sess, err := backend.TelegramAuth(ctx, user.ExternalID, user.LanguageCode, user.Username, user.FirstName, user.ServiceLanguage)
+		sess, err := backend.TelegramAuth(ctx, user.ExternalID, user.LanguageCode, user.Username, user.FirstName)
 		if err != nil {
 			return nil, err
 		}
-		return encodeSession(sess, user.SupportedLanguages), nil
+		return encodeSession(sess), nil
 	}
 }
 
-func authGuestHandler(backend *backendclient.Client, supportedLangs []string) Handler {
+func authGuestHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, _ Request) ([]byte, error) {
 		sess, err := backend.GuestAuth(ctx)
 		if err != nil {
 			return nil, err
 		}
-		return encodeSession(sess, supportedLangs), nil
+		return encodeSession(sess), nil
 	}
 }
 
@@ -192,14 +186,14 @@ func authEmailRequestHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-func authEmailLoginHandler(backend *backendclient.Client, supportedLangs []string) Handler {
+func authEmailLoginHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsEmailLoginRequest(req.Payload, 0)
 		sess, err := backend.EmailLogin(ctx, string(in.Email()), string(in.Code()))
 		if err != nil {
 			return nil, err
 		}
-		return encodeSession(sess, supportedLangs), nil
+		return encodeSession(sess), nil
 	}
 }