fix: downloaded episodes no longer appear as missing

Use the database as the authoritative source for missing-episode lists so
that episodes marked is_downloaded=True are never shown as missing, even
when the in-memory state is stale.

Key changes:
- EpisodeService.get_by_series() gains only_missing flag
- AnimeService uses DB-backed episodeDict and preserves downloaded episodes
  during sync, skipping them when adding/removing missing episodes
- DownloadService broadcasts series_updated after marking an episode downloaded
  so the frontend reflects the change immediately
- Frontend filters out series with zero missing episodes client-side and
  fixes renderSeries to respect the active filter
- Unit tests updated to assert the broadcast is sent

tests/unit/test_download_service.py

@@ -101,6 +101,7 @@ def mock_anime_service():
     service = MagicMock(spec=AnimeService)
     service.download = AsyncMock(return_value=True)
     service._directory = "/mock/anime/directory"
+    service._broadcast_series_updated = AsyncMock(return_value=None)
     return service
 
 
@@ -848,6 +849,10 @@ class TestRemoveEpisodeFromMissingList:
         assert serie.episodeDict[1] == [1, 3]
         # Cache was cleared
         download_service._anime_service._cached_list_missing.cache_clear.assert_called()
+        # Broadcast was sent so frontend gets real-time update
+        download_service._anime_service._broadcast_series_updated.assert_awaited_once_with(
+            "test-series"
+        )
         assert result is True
 
     @pytest.mark.asyncio