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/feedback/service.go

@@ -112,14 +112,9 @@ func (svc *Service) Submit(ctx context.Context, accountID uuid.UUID, body string
 		attachmentName = "" // a name without bytes carries no attachment
 	}
 	ch := normalizeChannel(channel)
-	// Snapshot the languages at submit time (acc is already loaded for the guest check):
-	// the sender's interface language, and the connector bot language when the message
-	// came through an external connector (currently Telegram).
-	var channelLang string
-	if ch == "telegram" {
-		channelLang = acc.ServiceLanguage
-	}
-	_, err = svc.store.Insert(ctx, accountID, body, attachment, attachmentName, ch, acc.PreferredLanguage, channelLang, parseIP(senderIP))
+	// Snapshot the sender's interface language at submit time (acc is already loaded
+	// for the guest check) so the operator later sees the state as it was.
+	_, err = svc.store.Insert(ctx, accountID, body, attachment, attachmentName, ch, acc.PreferredLanguage, parseIP(senderIP))
 	return err
 }