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/landing.ts

@@ -1,20 +1,15 @@
-// Pure helpers for the public landing page, kept out of the Svelte component so
-// the per-language Telegram-channel link selection is unit-testable.
-
-import type { Locale } from './i18n/index.svelte';
+// Pure helpers for the public landing page, kept out of the Svelte component so the
+// Telegram-channel link selection is unit-testable.
 
 /**
- * telegramChannelLink returns the t.me link for the locale's game channel, or null when it is
- * not configured. The channel usernames are build-time vars (VITE_TELEGRAM_GAME_CHANNEL_NAME_EN
- * / VITE_TELEGRAM_GAME_CHANNEL_NAME_RU) because the test and prod contours run different
- * channels; they are the same channels the connector posts to via TELEGRAM_GAME_CHANNEL_ID_*
+ * telegramChannelLink returns the t.me link for the single bot's game channel, or null
+ * when it is not configured. The channel username is a build-time var
+ * (VITE_TELEGRAM_GAME_CHANNEL_NAME) because the test and prod contours run different
+ * channels; it is the same channel the connector posts to via TELEGRAM_GAME_CHANNEL_ID
  * (the id to post, the name to link). A leading "@" is tolerated.
  */
-export function telegramChannelLink(locale: Locale): string | null {
-  const raw =
-    locale === 'ru'
-      ? import.meta.env.VITE_TELEGRAM_GAME_CHANNEL_NAME_RU
-      : import.meta.env.VITE_TELEGRAM_GAME_CHANNEL_NAME_EN;
+export function telegramChannelLink(): string | null {
+  const raw = import.meta.env.VITE_TELEGRAM_GAME_CHANNEL_NAME;
   const name = (raw as string | undefined)?.trim().replace(/^@/, '');
   return name ? `https://t.me/${name}` : null;
 }