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.

ui/src/lib/codec.test.ts

@@ -47,17 +47,20 @@ describe('codec', () => {
     const token = b.createString('tok');
     const uid = b.createString('u1');
     const name = b.createString('Me');
+    const langs = fb.Session.createSupportedLanguagesVector(b, [b.createString('en'), b.createString('ru')]);
     fb.Session.startSession(b);
     fb.Session.addToken(b, token);
     fb.Session.addUserId(b, uid);
     fb.Session.addIsGuest(b, true);
     fb.Session.addDisplayName(b, name);
+    fb.Session.addSupportedLanguages(b, langs);
     b.finish(fb.Session.endSession(b));
     expect(decodeSession(b.asUint8Array())).toEqual({
       token: 'tok',
       userId: 'u1',
       isGuest: true,
       displayName: 'Me',
+      supportedLanguages: ['en', 'ru'],
     });
   });
 
@@ -181,7 +184,7 @@ describe('codec', () => {
     b.finish(fb.LinkResult.endLinkResult(b));
     const r = decodeLinkResult(b.asUint8Array());
     expect(r.status).toBe('merged');
-    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya' });
+    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya', supportedLanguages: [] });
   });
 
   it('decodes an Invitation with inviter and invitees', () => {