feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

ui/src/lib/deeplink.test.ts

@@ -35,19 +35,6 @@ describe('shareLink', () => {
     vi.stubEnv('VITE_TELEGRAM_LINK', 'https://t.me/bot/app');
     expect(shareLink('f123456')).toBe('https://t.me/bot/app?startapp=f123456');
   });
-
-  it('picks the per-bot base by language', () => {
-    vi.stubEnv('VITE_TELEGRAM_LINK_EN', 'https://t.me/enbot/app');
-    vi.stubEnv('VITE_TELEGRAM_LINK_RU', 'https://t.me/rubot/app');
-    expect(shareLink('f1', 'en')).toBe('https://t.me/enbot/app?startapp=f1');
-    expect(shareLink('f1', 'ru')).toBe('https://t.me/rubot/app?startapp=f1');
-  });
-
-  it('falls back to the language-agnostic base when the per-bot one is unset', () => {
-    vi.stubEnv('VITE_TELEGRAM_LINK', 'https://t.me/fallback/app');
-    vi.stubEnv('VITE_TELEGRAM_LINK_RU', '');
-    expect(shareLink('f1', 'ru')).toBe('https://t.me/fallback/app?startapp=f1');
-  });
 });
 
 describe('botUsername', () => {
@@ -62,11 +49,4 @@ describe('botUsername', () => {
     vi.stubEnv('VITE_TELEGRAM_LINK', 'https://t.me/bot/app');
     expect(botUsername()).toBe('bot');
   });
-
-  it('picks the per-bot handle by language', () => {
-    vi.stubEnv('VITE_TELEGRAM_LINK_EN', 'https://t.me/enbot/app');
-    vi.stubEnv('VITE_TELEGRAM_LINK_RU', 'https://t.me/rubot/app');
-    expect(botUsername('en')).toBe('enbot');
-    expect(botUsername('ru')).toBe('rubot');
-  });
 });