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/variants.test.ts

@@ -7,22 +7,29 @@ import {
   multipleWordsForRequest,
 } from './variants';
 
+describe('ALL_VARIANTS', () => {
+  it('lists variants Erudit-first', () => {
+    expect(ALL_VARIANTS.map((v) => v.id)).toEqual(['erudit_ru', 'scrabble_ru', 'scrabble_en']);
+  });
+});
+
 describe('availableVariants', () => {
-  it('is ungated (all variants) for an empty or absent set', () => {
+  it('is ungated (all variants) for an empty or absent preference 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(['scrabble_en']);
+  it('offers only the preferred variants', () => {
+    expect(availableVariants(['scrabble_en']).map((v) => v.id)).toEqual(['scrabble_en']);
+    expect(availableVariants(['erudit_ru', 'scrabble_en']).map((v) => v.id)).toEqual(['erudit_ru', 'scrabble_en']);
   });
 
-  it('offers Russian and Эрудит for a ru-only service', () => {
-    expect(availableVariants(['ru']).map((v) => v.id)).toEqual(['scrabble_ru', 'erudit_ru']);
-  });
-
-  it('offers every variant for a bilingual service', () => {
-    expect(availableVariants(['en', 'ru']).map((v) => v.id)).toEqual(['scrabble_en', 'scrabble_ru', 'erudit_ru']);
+  it('keeps the Erudit-first catalogue order regardless of preference order', () => {
+    expect(availableVariants(['scrabble_en', 'erudit_ru', 'scrabble_ru']).map((v) => v.id)).toEqual([
+      'erudit_ru',
+      'scrabble_ru',
+      'scrabble_en',
+    ]);
   });
 });