Stage 15: dual Telegram bots & language-gated variants

Service-agnostic refinement of the owner's idea: the sign-in service returns a
set of supported game languages with the user identity, and the lobby gates the
New Game variant choice by it (en -> English; ru -> Russian + Эрудит).

- Connector hosts two bots in one container (one per service language, each its
  own token + game channel; the same telegram_id spans both). ValidateInitData
  tries each token and returns the validating bot's service_language +
  supported_languages. Per-language config (TELEGRAM_BOT_TOKEN_EN/_RU, channels).
- supported_languages rides the Session (fbs, session-scoped, not persisted); the
  UI offers only the matching variants on New Game — gating only the START of a
  new game (auto-match + friend invite), not accept/open/play; backend does not
  enforce.
- service_language persisted (accounts.service_language, migration 00010, written
  every login, last-login-wins) and routes the user-facing Notify push back
  through the right bot (push-target coalesces with preferred_language).
- Admin SendToUser/SendToGameChannel gain an operator-chosen language selector in
  the console (unrelated to ValidateInitData).
- Non-Telegram logins carry the gateway default set
  (GATEWAY_DEFAULT_SUPPORTED_LANGUAGES, all variants).

Wire (committed regen): ValidateInitDataResponse +service_language
+supported_languages; Session +supported_languages; SendToUser/SendToGameChannel
+language. Docs (ARCHITECTURE/FUNCTIONAL/_ru/READMEs) + PLAN updated; stage marked done.

backend/internal/account/account.go

@@ -56,6 +56,12 @@ 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
 	// IsGuest marks an ephemeral guest account: a durable row with no identity,
 	// excluded from statistics, friends and history.
 	IsGuest bool
@@ -374,16 +380,41 @@ func (s *Store) SpendHint(ctx context.Context, id uuid.UUID) (bool, error) {
 	return n > 0, 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
+	}
 	return Account{
 		ID:                     row.AccountID,
 		DisplayName:            row.DisplayName,
 		PreferredLanguage:      row.PreferredLanguage,
+		ServiceLanguage:        serviceLanguage,
 		TimeZone:               row.TimeZone,
 		AwayStart:              row.AwayStart,
 		AwayEnd:                row.AwayEnd,