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/web/static/js/index/series-manager.js

@@ -203,6 +203,17 @@ AniWorld.SeriesManager = (function() {
     function applyFiltersAndSort() {
         let filtered = seriesData.slice();
 
+        // Apply client-side filter so that real-time WebSocket updates
+        // (e.g. an episode being marked downloaded) are immediately
+        // reflected without a full server reload.
+        if (filterMode === 'missing_episodes') {
+            filtered = filtered.filter(function(s) {
+                return s.missing_episodes > 0;
+            });
+        }
+        // 'no_episodes' filter state is maintained server-side;
+        // don't try to replicate it client-side here.
+
         // Sort based on the current sorting mode
         filtered.sort(function(a, b) {
             if (sortAlphabetical) {
@@ -233,8 +244,12 @@ AniWorld.SeriesManager = (function() {
      */
     function renderSeries() {
         const grid = document.getElementById('series-grid');
-        const dataToRender = filteredSeriesData.length > 0 ? filteredSeriesData :
-            (seriesData.length > 0 ? seriesData : []);
+        // Always use filteredSeriesData — applyFiltersAndSort() is always
+        // called before renderSeries(), so filteredSeriesData is current.
+        // The old fallback to seriesData was incorrect: when a filter is
+        // active and filteredSeriesData is empty it must show the empty-state
+        // message, not fall through to unfiltered seriesData.
+        const dataToRender = filteredSeriesData;
 
         if (dataToRender.length === 0) {
             let message;