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/game/service.go

@@ -554,16 +554,6 @@ func (svc *Service) GameVariant(ctx context.Context, gameID uuid.UUID) (engine.V
 	return svc.store.GetGameVariant(ctx, gameID)
 }
 
-// GameLanguage returns the game's language tag ("en"/"ru"), derived from its variant, so a
-// game push routes out-of-app to the game's own bot rather than the recipient's last-login bot.
-func (svc *Service) GameLanguage(ctx context.Context, gameID uuid.UUID) (string, error) {
-	v, err := svc.GameVariant(ctx, gameID)
-	if err != nil {
-		return "", err
-	}
-	return v.Language(), nil
-}
-
 // RobotSchedule returns a game's bag seed and turn-start time, for the admin console's
 // robot-schedule panel (the deterministic play-to-win intent and next-move ETA).
 func (svc *Service) RobotSchedule(ctx context.Context, gameID uuid.UUID) (seed int64, turnStartedAt time.Time, err error) {
@@ -740,9 +730,6 @@ func (svc *Service) emitMove(ctx context.Context, post Game, rec engine.MoveReco
 		}
 		intents = append(intents, notify.OpponentMoved(s.AccountID, post.ID, rec, summary, bagLen))
 	}
-	// Game pushes are routed out-of-app by the game's own language, not the recipient's
-	// last-login bot.
-	lang := post.Variant.Language()
 	switch post.Status {
 	case StatusActive:
 		// Honest-AI games suppress your_turn: the robot replies instantly, so a "your turn"
@@ -757,7 +744,6 @@ func (svc *Service) emitMove(ctx context.Context, post Game, rec engine.MoveReco
 			}
 			opponent := svc.displayName(ctx, post.Seats, rec.Player)
 			yourTurn := notify.YourTurn(next, post.ID, deadline, opponent, action, word, scoreLine(post, post.ToMove), post.MoveCount)
-			yourTurn.Language = lang
 			intents = append(intents, yourTurn)
 		}
 	case StatusFinished:
@@ -769,7 +755,6 @@ func (svc *Service) emitMove(ctx context.Context, post Game, rec engine.MoveReco
 				continue
 			}
 			over := notify.GameOver(s.AccountID, post.ID, seatResult(post.Seats, s.Seat), scoreLine(post, s.Seat), summary)
-			over.Language = lang
 			intents = append(intents, over)
 		}
 	}