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.

backend/internal/server/handlers_auth.go

@@ -19,20 +19,16 @@ import (
 // telegramAuthRequest carries the identity the connector extracted from a
 // validated initData payload. Username, FirstName and LanguageCode seed a
 // brand-new account's display name and language (first contact only).
-// ServiceLanguage is the validating bot's language tag (en/ru); it is recorded on
-// every login (the bot the user last came through) and routes their out-of-app push.
 type telegramAuthRequest struct {
-	ExternalID      string `json:"external_id"`
-	Username        string `json:"username"`
-	FirstName       string `json:"first_name"`
-	LanguageCode    string `json:"language_code"`
-	ServiceLanguage string `json:"service_language"`
+	ExternalID   string `json:"external_id"`
+	Username     string `json:"username"`
+	FirstName    string `json:"first_name"`
+	LanguageCode string `json:"language_code"`
 }
 
 // handleTelegramAuth provisions (or finds) the account bound to a Telegram
 // identity and mints a session for it, seeding a new account's display name and
-// language from the supplied Telegram fields and recording the validating bot's
-// service language (updated every login) so out-of-app push routes to that bot.
+// language from the supplied Telegram fields (first contact only).
 func (s *Server) handleTelegramAuth(c *gin.Context) {
 	var req telegramAuthRequest
 	if err := c.ShouldBindJSON(&req); err != nil || req.ExternalID == "" {
@@ -44,11 +40,6 @@ func (s *Server) handleTelegramAuth(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	if err := s.accounts.SetServiceLanguage(c.Request.Context(), acc.ID, req.ServiceLanguage); err != nil {
-		s.abortErr(c, err)
-		return
-	}
-	acc.ServiceLanguage = req.ServiceLanguage // reflect this login's bot in the session response
 	s.mintSession(c, acc)
 }
 
@@ -59,10 +50,9 @@ type pushTargetRequest struct {
 
 // pushTargetResponse carries what the gateway needs to route an out-of-app push:
 // the recipient's Telegram external_id (empty when they have no Telegram
-// identity, e.g. a guest or email-only account), the language that both selects the
-// delivering bot and renders the message (the account's service language, the bot
-// it last signed in through, falling back to its preferred language), and whether
-// they confined notifications to the in-app stream.
+// identity, e.g. a guest or email-only account), the language the single bot renders
+// the message in (the account's interface language), and whether they confined
+// notifications to the in-app stream.
 type pushTargetResponse struct {
 	ExternalID             string `json:"external_id"`
 	Language               string `json:"language"`
@@ -94,15 +84,9 @@ func (s *Server) handlePushTarget(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	// Route by the bot the user last signed in through; fall back to the interface
-	// language for an account that has never come through a tagged bot.
-	language := acc.ServiceLanguage
-	if language == "" {
-		language = acc.PreferredLanguage
-	}
 	c.JSON(http.StatusOK, pushTargetResponse{
 		ExternalID:             ext,
-		Language:               language,
+		Language:               acc.PreferredLanguage,
 		NotificationsInAppOnly: acc.NotificationsInAppOnly,
 	})
 }