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_account.go

@@ -18,14 +18,15 @@ import (
 // updateProfileRequest is the full editable profile. away_start/away_end are
 // "HH:MM" local-time bounds of the daily away window.
 type updateProfileRequest struct {
-	DisplayName            string `json:"display_name"`
-	PreferredLanguage      string `json:"preferred_language"`
-	TimeZone               string `json:"time_zone"`
-	AwayStart              string `json:"away_start"`
-	AwayEnd                string `json:"away_end"`
-	BlockChat              bool   `json:"block_chat"`
-	BlockFriendRequests    bool   `json:"block_friend_requests"`
-	NotificationsInAppOnly bool   `json:"notifications_in_app_only"`
+	DisplayName            string   `json:"display_name"`
+	PreferredLanguage      string   `json:"preferred_language"`
+	TimeZone               string   `json:"time_zone"`
+	AwayStart              string   `json:"away_start"`
+	AwayEnd                string   `json:"away_end"`
+	BlockChat              bool     `json:"block_chat"`
+	BlockFriendRequests    bool     `json:"block_friend_requests"`
+	NotificationsInAppOnly bool     `json:"notifications_in_app_only"`
+	VariantPreferences     []string `json:"variant_preferences"`
 }
 
 // statsDTO is a durable account's lifetime statistics (the derived games-played and
@@ -92,6 +93,7 @@ func (s *Server) handleUpdateProfile(c *gin.Context) {
 		BlockChat:              req.BlockChat,
 		BlockFriendRequests:    req.BlockFriendRequests,
 		NotificationsInAppOnly: req.NotificationsInAppOnly,
+		VariantPreferences:     req.VariantPreferences,
 	})
 	if err != nil {
 		s.abortErr(c, err)