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.

deploy/docker-compose.yml

@@ -121,10 +121,7 @@ services:
       args:
         VITE_TELEGRAM_BOT_ID: ${VITE_TELEGRAM_BOT_ID:-}
         VITE_TELEGRAM_LINK: ${VITE_TELEGRAM_LINK:-}
-        VITE_TELEGRAM_LINK_EN: ${VITE_TELEGRAM_LINK_EN:-}
-        VITE_TELEGRAM_LINK_RU: ${VITE_TELEGRAM_LINK_RU:-}
-        VITE_TELEGRAM_GAME_CHANNEL_NAME_EN: ${VITE_TELEGRAM_GAME_CHANNEL_NAME_EN:-}
-        VITE_TELEGRAM_GAME_CHANNEL_NAME_RU: ${VITE_TELEGRAM_GAME_CHANNEL_NAME_RU:-}
+        VITE_TELEGRAM_GAME_CHANNEL_NAME: ${VITE_TELEGRAM_GAME_CHANNEL_NAME:-}
         VITE_GATEWAY_URL: ${VITE_GATEWAY_URL:-}
         VITE_APP_VERSION: ${APP_VERSION:-dev}
     restart: unless-stopped
@@ -135,7 +132,6 @@ services:
       GATEWAY_BACKEND_HTTP_URL: http://backend:8080
       GATEWAY_BACKEND_GRPC_ADDR: backend:9090
       GATEWAY_CONNECTOR_ADDR: telegram:9091
-      GATEWAY_DEFAULT_SUPPORTED_LANGUAGES: ${GATEWAY_DEFAULT_SUPPORTED_LANGUAGES:-en,ru}
       GATEWAY_LOG_LEVEL: ${LOG_LEVEL:-info}
       GATEWAY_SERVICE_NAME: scrabble-gateway
       GATEWAY_OTEL_TRACES_EXPORTER: otlp
@@ -171,10 +167,7 @@ services:
       args:
         VITE_TELEGRAM_BOT_ID: ${VITE_TELEGRAM_BOT_ID:-}
         VITE_TELEGRAM_LINK: ${VITE_TELEGRAM_LINK:-}
-        VITE_TELEGRAM_LINK_EN: ${VITE_TELEGRAM_LINK_EN:-}
-        VITE_TELEGRAM_LINK_RU: ${VITE_TELEGRAM_LINK_RU:-}
-        VITE_TELEGRAM_GAME_CHANNEL_NAME_EN: ${VITE_TELEGRAM_GAME_CHANNEL_NAME_EN:-}
-        VITE_TELEGRAM_GAME_CHANNEL_NAME_RU: ${VITE_TELEGRAM_GAME_CHANNEL_NAME_RU:-}
+        VITE_TELEGRAM_GAME_CHANNEL_NAME: ${VITE_TELEGRAM_GAME_CHANNEL_NAME:-}
         VITE_GATEWAY_URL: ${VITE_GATEWAY_URL:-}
         VITE_APP_VERSION: ${APP_VERSION:-dev}
     restart: unless-stopped
@@ -209,12 +202,11 @@ services:
     depends_on: [vpn]
     network_mode: "service:vpn"
     environment:
-      # The bot tokens live ONLY in this container (ARCHITECTURE.md §12). At least
-      # one token is required (the connector validates this at boot).
-      TELEGRAM_BOT_TOKEN_EN: ${TELEGRAM_BOT_TOKEN_EN:-}
-      TELEGRAM_BOT_TOKEN_RU: ${TELEGRAM_BOT_TOKEN_RU:-}
-      TELEGRAM_GAME_CHANNEL_ID_EN: ${TELEGRAM_GAME_CHANNEL_ID_EN:-}
-      TELEGRAM_GAME_CHANNEL_ID_RU: ${TELEGRAM_GAME_CHANNEL_ID_RU:-}
+      # The bot token lives ONLY in this container (ARCHITECTURE.md §12). The connector
+      # requires it at boot; an empty value leaves the Telegram side down while the rest
+      # of the contour still comes up.
+      TELEGRAM_BOT_TOKEN: ${TELEGRAM_BOT_TOKEN:-}
+      TELEGRAM_GAME_CHANNEL_ID: ${TELEGRAM_GAME_CHANNEL_ID:-}
       TELEGRAM_MINIAPP_URL: ${TELEGRAM_MINIAPP_URL:?set TELEGRAM_MINIAPP_URL}
       TELEGRAM_GRPC_ADDR: ":9091"
       TELEGRAM_TEST_ENV: ${TELEGRAM_TEST_ENV:-false}