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/backendclient/api.go

@@ -146,15 +146,18 @@ type ChatResp struct {
 }
 
 // TelegramAuth provisions/finds the Telegram account and mints a session, seeding a
-// brand-new account's display name and language from the validated launch fields.
-func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName string) (SessionResp, error) {
+// brand-new account's display name and language from the validated launch fields and
+// recording the validating bot's serviceLanguage (which routes the account's later
+// out-of-app push).
+func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName, serviceLanguage string) (SessionResp, error) {
 	var out SessionResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/internal/sessions/telegram", "", "",
 		map[string]string{
-			"external_id":   externalID,
-			"language_code": languageCode,
-			"username":      username,
-			"first_name":    firstName,
+			"external_id":      externalID,
+			"language_code":    languageCode,
+			"username":         username,
+			"first_name":       firstName,
+			"service_language": serviceLanguage,
 		}, &out)
 	return out, err
 }