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/services/download_service.py

@@ -275,7 +275,7 @@ class DownloadService:
         """
         try:
             from src.server.database.connection import get_db_session
-            from src.server.database.service import EpisodeService, AnimeSeriesService
+            from src.server.database.service import AnimeSeriesService, EpisodeService
             
             logger.info(
                 "Attempting to mark episode as downloaded in DB: "
@@ -362,6 +362,31 @@ class DownloadService:
             except Exception:
                 pass
             
+            # Broadcast real-time update to frontend so the series card
+            # immediately reflects the new downloaded state (no longer
+            # shows the episode as missing) without waiting for a full
+            # reload on DOWNLOAD_COMPLETED.
+            try:
+                await self._anime_service._broadcast_series_updated(
+                    series_key
+                )
+                logger.debug(
+                    "Broadcast series_updated after marking "
+                    "%s S%02dE%02d as downloaded",
+                    series_key,
+                    season,
+                    episode,
+                )
+            except Exception as broadcast_exc:
+                logger.warning(
+                    "Failed to broadcast series update after marking "
+                    "%s S%02dE%02d as downloaded: %s",
+                    series_key,
+                    season,
+                    episode,
+                    broadcast_exc,
+                )
+            
             return True
         except Exception as e:
             logger.error(