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/social/social.go

@@ -44,9 +44,6 @@ type GameReader interface {
 	// one-chat-message-per-turn limit (a message counts toward the current turn when it
 	// was posted at or after this time).
 	TurnStartedAt(ctx context.Context, gameID uuid.UUID) (time.Time, error)
-	// GameLanguage is the game's language tag ("en"/"ru"), so a nudge's out-of-app push routes
-	// to the game's bot rather than the recipient's last-login bot.
-	GameLanguage(ctx context.Context, gameID uuid.UUID) (string, error)
 	// VsAI reports whether the game is an honest-AI game, in which chat and nudge are
 	// both disabled (the game still reports status 'active').
 	VsAI(ctx context.Context, gameID uuid.UUID) (bool, error)