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
Lukas
9a81b04b65