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/account/account.go

@@ -55,12 +55,12 @@ type Account struct {
 	HintBalance         int
 	BlockChat           bool
 	BlockFriendRequests bool
-	// ServiceLanguage is the language tag (en/ru) of the bot the account last
-	// authenticated through (its last Telegram ValidateInitData); it routes the
-	// account's out-of-app push back through the right bot. Empty when the account
-	// has never signed in through a tagged bot. Distinct from PreferredLanguage (the
-	// interface language) and from a game's variant language.
-	ServiceLanguage string
+	// VariantPreferences is the set of game variants (engine.Variant stable labels:
+	// "scrabble_en", "scrabble_ru", "erudit_ru") the player is willing to be matched
+	// into. It gates the New Game picker, the matchmaker and the friend-invite the
+	// player creates; an invited friend may still accept any variant. A new account
+	// defaults to Erudit only. Never empty — enforced on update and by a DB check.
+	VariantPreferences []string
 	// IsGuest marks an ephemeral guest account: a durable row with no identity,
 	// excluded from statistics, friends and history.
 	IsGuest bool
@@ -491,36 +491,12 @@ func (s *Store) ClearHighRateFlag(ctx context.Context, id uuid.UUID) error {
 	return nil
 }
 
-// SetServiceLanguage records the service language (en/ru) of the bot a Telegram
-// user authenticated through. It is called on every Telegram login — new and
-// existing accounts — so it tracks the bot the user last came through (last-login-
-// wins), and the out-of-app push routes by it. It is a no-op for an empty language
-// (a non-Telegram login carries none) and does not bump updated_at (an infra
-// routing field, not a user profile edit).
-func (s *Store) SetServiceLanguage(ctx context.Context, id uuid.UUID, language string) error {
-	if language == "" {
-		return nil
-	}
-	stmt := table.Accounts.
-		UPDATE(table.Accounts.ServiceLanguage).
-		SET(postgres.String(language)).
-		WHERE(table.Accounts.AccountID.EQ(postgres.UUID(id)))
-	if _, err := stmt.ExecContext(ctx, s.db); err != nil {
-		return fmt.Errorf("account: set service language %s: %w", id, err)
-	}
-	return nil
-}
-
 // modelToAccount projects a generated model row into the public Account struct.
 func modelToAccount(row model.Accounts) Account {
 	var mergedInto uuid.UUID
 	if row.MergedInto != nil {
 		mergedInto = *row.MergedInto
 	}
-	var serviceLanguage string
-	if row.ServiceLanguage != nil {
-		serviceLanguage = *row.ServiceLanguage
-	}
 	var flaggedHighRateAt time.Time
 	if row.FlaggedHighRateAt != nil {
 		flaggedHighRateAt = *row.FlaggedHighRateAt
@@ -529,7 +505,7 @@ func modelToAccount(row model.Accounts) Account {
 		ID:                     row.AccountID,
 		DisplayName:            row.DisplayName,
 		PreferredLanguage:      row.PreferredLanguage,
-		ServiceLanguage:        serviceLanguage,
+		VariantPreferences:     []string(row.VariantPreferences),
 		TimeZone:               row.TimeZone,
 		AwayStart:              row.AwayStart,
 		AwayEnd:                row.AwayEnd,