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/components/StaleInviteModal.svelte

@@ -5,9 +5,8 @@
   import { telegramOpenLink } from '../lib/telegram';
   import Modal from './Modal.svelte';
 
-  // Point at the bot the player signed in through (its service language), falling back to
-  // the interface locale, so an ru player is sent to the ru bot and an en player to the en one.
-  const username = $derived(botUsername(app.session?.serviceLanguage || app.locale));
+  // The single bot's @username, for the deep link.
+  const username = $derived(botUsername());
   // Split the message around the {bot} token so the bot handle renders as an inline link.
   const parts = $derived(t('friends.staleInvite').split('{bot}'));
 

ui/src/components/WelcomeRedeemModal.svelte

@@ -5,9 +5,8 @@
   import { telegramOpenLink } from '../lib/telegram';
   import Modal from './Modal.svelte';
 
-  // Point at the bot the player signed in through (its service language), falling back to the
-  // interface locale, so an ru player is sent to the ru bot and an en player to the en one.
-  const username = $derived(botUsername(app.session?.serviceLanguage || app.locale));
+  // The single bot's @username, for the deep link.
+  const username = $derived(botUsername());
   // Greet the arriving player by their own display name.
   const name = $derived(app.profile?.displayName || app.session?.displayName || '');
   // Interpolate the name, then split the rest around the {bot} token so the bot handle renders