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.

gateway/README.md

@@ -77,7 +77,6 @@ connector (`ValidateLoginWidget`) and forward the trusted `external_id`. These
 | `GATEWAY_BACKEND_TIMEOUT` | `5s` | per backend REST call |
 | `GATEWAY_ADMIN_USER` / `GATEWAY_ADMIN_PASSWORD` | unset | enable + guard the admin console at `/_gm` |
 | `GATEWAY_CONNECTOR_ADDR` | unset | Telegram connector gRPC address (enables initData validation + out-of-app push) |
-| `GATEWAY_DEFAULT_SUPPORTED_LANGUAGES` | `en,ru` | New Game variant gating set placed on the Session for non-platform logins (web / email / guest); a deployment may narrow it |
 | `GATEWAY_SESSION_TTL` | `10m` | cached session lifetime |
 | `GATEWAY_SESSION_CACHE_MAX` | `50000` | cached session cap |
 | `GATEWAY_PUSH_HEARTBEAT_INTERVAL` | `10s` | live-stream keep-alive (an immediate heartbeat also fires on open, under the ~15s edge idle timeout) |