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.

pkg/fbs/scrabble.fbs

@@ -123,20 +123,11 @@ table EmailLoginRequest {
 }
 
 // Session is the minted credential returned by every auth operation.
-// supported_languages is the set of game languages (subset of {en, ru}, at least
-// one) the service the user signed in through offers; the UI gates the New Game
-// variant choice by it (en -> English; ru -> Russian + Эрудит). It is session-
-// scoped (not persisted) and added trailing — backward-compatible.
 table Session {
   token:string;
   user_id:string;
   is_guest:bool;
   display_name:string;
-  supported_languages:[string];
-  // service_language is the language tag (en/ru) of the Telegram bot the session was
-  // minted through; empty for a non-Telegram login. The UI uses it to build a share
-  // link that points at the same bot.
-  service_language:string;
 }
 
 // Ack is a simple success acknowledgement (e.g. an email-code request).
@@ -190,6 +181,9 @@ table Profile {
   away_end:string;
   notifications_in_app_only:bool = true;
   banner:BannerInfo;
+  // variant_preferences is the set of game variants the player allows themselves to be
+  // matched into (engine.Variant labels), Erudit-first; the New Game picker is gated by it.
+  variant_preferences:[string];
 }
 
 // BlockStatus reports the caller's current manual block. The UI fetches it after any operation
@@ -434,6 +428,7 @@ table UpdateProfileRequest {
   block_chat:bool;
   block_friend_requests:bool;
   notifications_in_app_only:bool = true;
+  variant_preferences:[string];
 }
 
 // --- account linking & merge (authenticated) ---