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.

ui/src/lib/model.ts

@@ -151,6 +151,9 @@ export interface Profile {
   isGuest: boolean;
   /** Confine notifications to the in-app stream (no out-of-app platform push). */
   notificationsInAppOnly: boolean;
+  /** Variants the player allows themselves to be matched into (Erudit-first). The New
+   *  Game picker is gated by this set, which is never empty. */
+  variantPreferences: Variant[];
   /** The advertising-banner block, present only for a viewer eligible to see it. */
   banner?: Banner;
 }
@@ -201,6 +204,7 @@ export interface ProfileUpdate {
   blockChat: boolean;
   blockFriendRequests: boolean;
   notificationsInAppOnly: boolean;
+  variantPreferences: Variant[];
 }
 
 /** A referenced account with its display name (friend, blocked user, invitee). */
@@ -327,14 +331,6 @@ export interface Session {
   userId: string;
   isGuest: boolean;
   displayName: string;
-  // supportedLanguages is the set of game languages the service the user signed in
-  // through offers (subset of {en, ru}, at least one). New Game offers only the
-  // variants these languages support (en -> English; ru -> Russian + Эрудит). Empty
-  // means ungated (all variants).
-  supportedLanguages: string[];
-  // serviceLanguage is the en/ru tag of the Telegram bot this session was minted
-  // through (empty for a non-Telegram login); used to share a link to the same bot.
-  serviceLanguage: string;
 }
 
 // LinkResult is the outcome of an account link/merge step. status is