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/inttest/game_limit_test.go

@@ -97,10 +97,12 @@ func TestGameLimitGate(t *testing.T) {
 	if !gamesListAtLimit(t, srv, human) {
 		t.Fatalf("at %d games at_game_limit must be true", game.MaxActiveQuickGames)
 	}
-	if rec := userPost(t, srv, "/api/v1/user/lobby/enqueue", human, `{"variant":"scrabble_en"}`); rec.Code != http.StatusConflict || errorCode(t, rec) != "game_limit_reached" {
+	// erudit_ru is in the default variant preferences, so the variant gate passes and the
+	// game-limit gate is what fires here.
+	if rec := userPost(t, srv, "/api/v1/user/lobby/enqueue", human, `{"variant":"erudit_ru"}`); rec.Code != http.StatusConflict || errorCode(t, rec) != "game_limit_reached" {
 		t.Fatalf("enqueue at limit = (%d, %q), want (409, game_limit_reached)", rec.Code, errorCode(t, rec))
 	}
-	invBody := fmt.Sprintf(`{"variant":"scrabble_en","invitee_ids":[%q]}`, opp.String())
+	invBody := fmt.Sprintf(`{"variant":"erudit_ru","invitee_ids":[%q]}`, opp.String())
 	if rec := userPost(t, srv, "/api/v1/user/invitations", human, invBody); rec.Code != http.StatusConflict || errorCode(t, rec) != "game_limit_reached" {
 		t.Fatalf("invitation at limit = (%d, %q), want (409, game_limit_reached)", rec.Code, errorCode(t, rec))
 	}