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.

docs/FUNCTIONAL.md

@@ -22,13 +22,17 @@ Settings also pick the board's bonus-label style (beginner / classic / none). A
 costs nothing when the rack has no legal move. The word-check accepts only the
 variant's alphabet, remembers answers within the session and rate-limits repeats.
 
-### Identity & sessions *(Stage 1 / 6 / 9)*
+### Identity & sessions *(Stage 1 / 6 / 9 / 15)*
 A player arrives from a platform (Telegram first), via email login, or as an
 ephemeral guest. The gateway validates the credential once and mints a thin
 session token; the backend resolves it to an internal `user_id`. A **Telegram Mini
 App** launch authenticates from the platform's signed `initData`, themes the UI to
 the Telegram colours, and — on first contact — seeds the new account's interface
-language from the Telegram client. Guests are session-only with restricted features
+language from the Telegram client. The sign-in service also declares the **game
+languages** it offers (a set of en/ru, at least one), which gate the New Game variant
+choice in the lobby. Telegram runs a separate bot per language (an English bot and a
+Russian bot, the same player spanning both); the bot a player signed in through both
+sets their offered languages and is the bot their out-of-app notifications come from. Guests are session-only with restricted features
 (auto-match only; no friends, stats or history); an abandoned guest that never
 joined a game and has been idle past the retention window is garbage-collected. While the app is open the client
 keeps a live stream and receives in-app updates in real time — the opponent's move,
@@ -49,10 +53,14 @@ when a guest links an identity that already has a durable account, where the dur
 account is kept and the guest's games move into it. A merge is blocked only while the
 two accounts share a game still in progress.
 
-### Lobby & matchmaking *(Stage 4)*
-Bottom tab menu: **my games**, **profile**. Auto-match (always 2 players) joins a
-per-variant pool and is paired with the next waiting human; after 10 s with no
-human the robot substitutes (the robot arrives in Stage 5). Friend games (2–4) are
+### Lobby & matchmaking *(Stage 4 / 15)*
+Bottom tab menu: **my games**, **profile**. The game types offered on **New Game** are
+limited to the languages the player's sign-in service supports (English → English;
+Russian → Russian + Эрудит; a bilingual service shows all three, and the web client is
+unrestricted). This gates only **starting** a new game — both auto-match and a friend
+invitation — so a player still sees and plays existing games of any language. Auto-match
+(always 2 players) joins a per-variant pool and is paired with the next waiting human;
+after 10 s with no human the robot substitutes (the robot arrives in Stage 5). Friend games (2–4) are
 formed by inviting players from the friend list (an invitation, like a friend code,
 is shareable as a Telegram deep link that opens it directly): the inviter chooses the
 settings and the game starts once every invitee has accepted — any decline cancels it, and an unanswered invitation