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.

gateway/internal/connector/client.go

@@ -27,12 +27,18 @@ var ErrInvalidInitData = errors.New("connector: invalid telegram init data")
 // rejects the Login Widget data (a gRPC InvalidArgument).
 var ErrInvalidLoginWidget = errors.New("connector: invalid telegram login widget data")
 
-// User is a validated Mini App identity.
+// User is a validated Mini App identity. ServiceLanguage is the validating bot's
+// language tag (en/ru), persisted to route the user's out-of-app push back through
+// the right bot; SupportedLanguages is that bot's set of offered game languages,
+// which the UI gates the New Game variant choice by. Both are empty for a Login
+// Widget validation (it carries no bot language).
 type User struct {
-	ExternalID   string
-	Username     string
-	FirstName    string
-	LanguageCode string
+	ExternalID         string
+	Username           string
+	FirstName          string
+	LanguageCode       string
+	ServiceLanguage    string
+	SupportedLanguages []string
 }
 
 // Client wraps the connector's Telegram gRPC service.
@@ -67,10 +73,12 @@ func (c *Client) ValidateInitData(ctx context.Context, initData string) (User, e
 		return User{}, err
 	}
 	return User{
-		ExternalID:   resp.GetExternalId(),
-		Username:     resp.GetUsername(),
-		FirstName:    resp.GetFirstName(),
-		LanguageCode: resp.GetLanguageCode(),
+		ExternalID:         resp.GetExternalId(),
+		Username:           resp.GetUsername(),
+		FirstName:          resp.GetFirstName(),
+		LanguageCode:       resp.GetLanguageCode(),
+		ServiceLanguage:    resp.GetServiceLanguage(),
+		SupportedLanguages: resp.GetSupportedLanguages(),
 	}, nil
 }