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

src/server/database/service.py

@@ -541,6 +541,7 @@ class EpisodeService:
         db: AsyncSession,
         series_id: int,
         season: Optional[int] = None,
+        only_missing: bool = False,
     ) -> List[Episode]:
         """Get episodes for a series.
         
@@ -548,6 +549,9 @@ class EpisodeService:
             db: Database session
             series_id: Foreign key to AnimeSeries
             season: Optional season filter
+            only_missing: If True, only return episodes where
+                is_downloaded is False (i.e., missing episodes).
+                Default False returns all episodes.
         
         Returns:
             List of Episode instances
@@ -557,6 +561,9 @@ class EpisodeService:
         if season is not None:
             query = query.where(Episode.season == season)
         
+        if only_missing:
+            query = query.where(Episode.is_downloaded == False)
+        
         query = query.order_by(Episode.season, Episode.episode_number)
         result = await db.execute(query)
         return list(result.scalars().all())