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.

.gitea/workflows/ci.yaml

@@ -254,25 +254,19 @@ jobs:
           AWG_CONF: ${{ secrets.TEST_AWG_CONF }}
           GM_BASICAUTH_HASH: ${{ secrets.TEST_GM_BASICAUTH_HASH }}
           GRAFANA_ADMIN_PASSWORD: ${{ secrets.TEST_GRAFANA_ADMIN_PASSWORD }}
-          TELEGRAM_BOT_TOKEN_EN: ${{ secrets.TEST_TELEGRAM_BOT_TOKEN_EN }}
-          TELEGRAM_BOT_TOKEN_RU: ${{ secrets.TEST_TELEGRAM_BOT_TOKEN_RU }}
+          TELEGRAM_BOT_TOKEN: ${{ secrets.TEST_TELEGRAM_BOT_TOKEN }}
           GM_BASICAUTH_USER: ${{ vars.TEST_GM_BASICAUTH_USER }}
           GRAFANA_ROOT_URL: ${{ vars.TEST_GRAFANA_ROOT_URL }}
           CADDY_SITE_ADDRESS: ${{ vars.TEST_CADDY_SITE_ADDRESS }}
           TELEGRAM_MINIAPP_URL: ${{ vars.TEST_TELEGRAM_MINIAPP_URL }}
-          TELEGRAM_GAME_CHANNEL_ID_EN: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID_EN }}
-          TELEGRAM_GAME_CHANNEL_ID_RU: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID_RU }}
+          TELEGRAM_GAME_CHANNEL_ID: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID }}
           # The test contour always uses Telegram's test environment — pinned here,
           # not an operator variable. The prod workflow leaves it false.
           TELEGRAM_TEST_ENV: "true"
           VITE_TELEGRAM_BOT_ID: ${{ vars.TEST_VITE_TELEGRAM_BOT_ID }}
           VITE_TELEGRAM_LINK: ${{ vars.TEST_VITE_TELEGRAM_LINK }}
-          VITE_TELEGRAM_LINK_EN: ${{ vars.TEST_VITE_TELEGRAM_LINK_EN }}
-          VITE_TELEGRAM_LINK_RU: ${{ vars.TEST_VITE_TELEGRAM_LINK_RU }}
-          VITE_TELEGRAM_GAME_CHANNEL_NAME_EN: ${{ vars.TEST_VITE_TELEGRAM_GAME_CHANNEL_NAME_EN }}
-          VITE_TELEGRAM_GAME_CHANNEL_NAME_RU: ${{ vars.TEST_VITE_TELEGRAM_GAME_CHANNEL_NAME_RU }}
+          VITE_TELEGRAM_GAME_CHANNEL_NAME: ${{ vars.TEST_VITE_TELEGRAM_GAME_CHANNEL_NAME }}
           VITE_GATEWAY_URL: ${{ vars.TEST_VITE_GATEWAY_URL }}
-          GATEWAY_DEFAULT_SUPPORTED_LANGUAGES: ${{ vars.TEST_GATEWAY_DEFAULT_SUPPORTED_LANGUAGES }}
           # Unset vars render empty -> the compose ":-" defaults apply.
           POSTGRES_DB: ${{ vars.TEST_POSTGRES_DB }}
           POSTGRES_USER: ${{ vars.TEST_POSTGRES_USER }}