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.

pkg/fbs/scrabble.fbs

@@ -109,11 +109,16 @@ table EmailLoginRequest {
 }
 
 // Session is the minted credential returned by every auth operation.
+// supported_languages is the set of game languages (subset of {en, ru}, at least
+// one) the service the user signed in through offers; the UI gates the New Game
+// variant choice by it (en -> English; ru -> Russian + Эрудит). It is session-
+// scoped (not persisted) and added trailing — backward-compatible.
 table Session {
   token:string;
   user_id:string;
   is_guest:bool;
   display_name:string;
+  supported_languages:[string];
 }
 
 // Ack is a simple success acknowledgement (e.g. an email-code request).