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.

backend/internal/game/emit_test.go

@@ -68,10 +68,6 @@ func TestEmitMoveNotifiesActor(t *testing.T) {
 	if got := string(yt.ScoreLine()); got != "13:19" { // seat 1 (recipient) first, then seat 0
 		t.Errorf("your_turn score_line = %q, want 13:19", got)
 	}
-	// Routed out-of-app by the game's language (the default Variant is English).
-	if yourTurn.Language != "en" {
-		t.Errorf("your_turn language = %q, want en", yourTurn.Language)
-	}
 }
 
 // TestEmitMoveAnnouncesGameOver checks the closing move sends a game_over push to every seat,
@@ -106,7 +102,4 @@ func TestEmitMoveAnnouncesGameOver(t *testing.T) {
 	if string(l.Result()) != "lost" || string(l.ScoreLine()) != "95:120" {
 		t.Errorf("loser game_over = %q / %q, want lost / 95:120", l.Result(), l.ScoreLine())
 	}
-	if over[winner].Language != "en" || over[loser].Language != "en" {
-		t.Errorf("game_over languages = %q/%q, want en/en", over[winner].Language, over[loser].Language)
-	}
 }