Stage 15: dual Telegram bots & language-gated variants

Service-agnostic refinement of the owner's idea: the sign-in service returns a
set of supported game languages with the user identity, and the lobby gates the
New Game variant choice by it (en -> English; ru -> Russian + Эрудит).

- Connector hosts two bots in one container (one per service language, each its
  own token + game channel; the same telegram_id spans both). ValidateInitData
  tries each token and returns the validating bot's service_language +
  supported_languages. Per-language config (TELEGRAM_BOT_TOKEN_EN/_RU, channels).
- supported_languages rides the Session (fbs, session-scoped, not persisted); the
  UI offers only the matching variants on New Game — gating only the START of a
  new game (auto-match + friend invite), not accept/open/play; backend does not
  enforce.
- service_language persisted (accounts.service_language, migration 00010, written
  every login, last-login-wins) and routes the user-facing Notify push back
  through the right bot (push-target coalesces with preferred_language).
- Admin SendToUser/SendToGameChannel gain an operator-chosen language selector in
  the console (unrelated to ValidateInitData).
- Non-Telegram logins carry the gateway default set
  (GATEWAY_DEFAULT_SUPPORTED_LANGUAGES, all variants).

Wire (committed regen): ValidateInitDataResponse +service_language
+supported_languages; Session +supported_languages; SendToUser/SendToGameChannel
+language. Docs (ARCHITECTURE/FUNCTIONAL/_ru/READMEs) + PLAN updated; stage marked done.

backend/README.md

@@ -80,8 +80,12 @@ the gateway fronts it with Basic-Auth and a same-origin guard protects its POSTs
 `resolved_at`/`applied_in_version` + the `status` CHECK) feeding a dictionary-change
 pipeline, dictionary **hot-reload** from `BACKEND_DICT_DIR/<version>/`
 (`engine.OpenWithVersions` / `Registry.LoadAvailable`), and operator **broadcasts** via a
-backend Telegram-connector client (`internal/connector`, `BACKEND_CONNECTOR_ADDR`). The
-shared wire contracts live in the sibling [`../pkg`](../pkg) module.
+backend Telegram-connector client (`internal/connector`, `BACKEND_CONNECTOR_ADDR`) — each
+broadcast picks the delivering bot by an operator-chosen language. **Stage 15** adds
+migration `00010` (`accounts.service_language`): the language tag of the bot a Telegram
+user last signed in through, written on every login and returned by
+`/internal/push-target` (falling back to `preferred_language`) so out-of-app push routes
+to the right bot. The shared wire contracts live in the sibling [`../pkg`](../pkg) module.
 
 Stage 11 adds **account linking & merge** (`/api/v1/user/link/*`). `internal/link`
 orchestrates it: an email confirm-code or a gateway-validated Telegram identity is