fix: exclude downloaded episodes from episodeDict and GetMissingEpisode

Prevent fully-downloaded series from being queued by auto-download:

- SerieList.GetMissingEpisode(): filter by is_downloaded instead of
  checking if episodeDict is non-empty. episodeDict from the DB
  relationship includes all episodes (including downloaded), so a
  series with only downloaded episodes still had a truthy episodeDict.

- AnimeSeries.episodeDict property: skip episodes where is_downloaded=True
  when building the dict from the DB relationship. This makes the
  property consistent with the is_downloaded filtering already done
  manually in list_series_with_filters(), and ensures that calling
  code anywhere in the codebase gets the correct missing-episode view.

Fixes hana-kimi (and any other fully-downloaded series) incorrectly
appearing in the auto-download queue after a rescan.

src/server/database/SerieList.py

@@ -134,7 +134,7 @@ class SerieList:
         """Return all series that still contain missing episodes."""
         return [
             anime for anime in self.keyDict.values()
-            if anime.episodeDict
+            if any(not ep.is_downloaded for ep in (anime.episodes or []))
         ]
 
     def get_missing_episodes(self) -> List[AnimeSeries]:

src/server/database/models.py

@@ -205,6 +205,8 @@ class AnimeSeries(Base, TimestampMixin):
         try:
             if self.episodes:
                 for ep in self.episodes:
+                    if ep.is_downloaded:
+                        continue
                     season = ep.season or 1
                     if season not in episode_dict:
                         episode_dict[season] = []

src/server/services/image_loading_service.py

@@ -162,17 +162,18 @@ class ImageLoadingService:
         for i in range(0, len(series_list), self.BATCH_SIZE):
             batch = series_list[i : i + self.BATCH_SIZE]
 
-            tasks = [
-                self.load_series_images(
+            # Process each series sequentially to avoid concurrent use of the
+            # same AsyncSession (SQLAlchemy async sessions are not thread-safe
+            # for concurrent operations). BATCH_SIZE still paces TMDB requests.
+            results: List[Dict[str, Any] | Exception] = []
+            for series in batch:
+                result = await self.load_series_images(
                     key=series["key"],
                     folder=series["folder"],
                     anime_directory=anime_directory,
                     db=db,
                 )
-                for series in batch
-            ]
-
-            results = await asyncio.gather(*tasks, return_exceptions=True)
+                results.append(result)
 
             for series, result in zip(batch, results):
                 if isinstance(result, Exception):