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/variants.test.ts

@@ -0,0 +1,22 @@
+import { describe, it, expect } from 'vitest';
+
+import { ALL_VARIANTS, availableVariants } from './variants';
+
+describe('availableVariants', () => {
+  it('is ungated (all variants) for an empty or absent set', () => {
+    expect(availableVariants([])).toEqual(ALL_VARIANTS);
+    expect(availableVariants(undefined)).toEqual(ALL_VARIANTS);
+  });
+
+  it('offers only English for an en-only service', () => {
+    expect(availableVariants(['en']).map((v) => v.id)).toEqual(['english']);
+  });
+
+  it('offers Russian and Эрудит for a ru-only service', () => {
+    expect(availableVariants(['ru']).map((v) => v.id)).toEqual(['russian', 'erudit']);
+  });
+
+  it('offers every variant for a bilingual service', () => {
+    expect(availableVariants(['en', 'ru']).map((v) => v.id)).toEqual(['english', 'russian', 'erudit']);
+  });
+});