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/deeplink.ts

@@ -41,25 +41,20 @@ function envVar(name: string): string | undefined {
 }
 
 /**
- * telegramBase returns the Mini App link base (e.g. https://t.me/<bot>/<app>) for a
- * bot language: VITE_TELEGRAM_LINK_EN / _RU when lang is en/ru, else the
- * language-agnostic VITE_TELEGRAM_LINK. Returns null when none is configured, so a
- * shared link points at the same bot the player signed in through.
+ * telegramBase returns the single bot's Mini App link base (e.g. https://t.me/<bot>/<app>)
+ * from VITE_TELEGRAM_LINK, or null when it is not configured.
  */
-function telegramBase(lang: string): string | null {
-  const byLang =
-    lang === 'ru' ? envVar('VITE_TELEGRAM_LINK_RU') : lang === 'en' ? envVar('VITE_TELEGRAM_LINK_EN') : undefined;
-  return byLang || envVar('VITE_TELEGRAM_LINK') || null;
+function telegramBase(): string | null {
+  return envVar('VITE_TELEGRAM_LINK') || null;
 }
 
 /**
  * botUsername extracts the bot's @username (without the @) from the configured Mini App
- * link for a bot language: the first path segment of https://t.me/<bot>/<app>. Returns
- * null when no base is configured or the link carries no path, so callers can fall back
- * when they cannot point at a specific bot.
+ * link: the first path segment of https://t.me/<bot>/<app>. Returns null when no base is
+ * configured or the link carries no path.
  */
-export function botUsername(lang = ''): string | null {
-  const base = telegramBase(lang);
+export function botUsername(): string | null {
+  const base = telegramBase();
   if (!base) return null;
   try {
     const seg = new URL(base).pathname.split('/').filter(Boolean)[0];
@@ -70,12 +65,11 @@ export function botUsername(lang = ''): string | null {
 }
 
 /**
- * shareLink wraps a deep-link start parameter in a t.me Mini App link for the given
- * bot language (the session's service language). Returns null when no base is
- * configured, so callers can hide the share affordance.
+ * shareLink wraps a deep-link start parameter in a t.me Mini App link for the single
+ * bot. Returns null when no base is configured, so callers can hide the share affordance.
  */
-export function shareLink(param: string, lang = ''): string | null {
-  const base = telegramBase(lang);
+export function shareLink(param: string): string | null {
+  const base = telegramBase();
   if (!base) return null;
   const sep = base.includes('?') ? '&' : '?';
   return `${base}${sep}startapp=${encodeURIComponent(param)}`;