fix: process image loading batch sequentially to avoid concurrent AsyncSession use

SQLAlchemy async sessions are not safe for concurrent operations. load_series_images_batch was using asyncio.gather to run multiple load_series_images calls concurrently, all sharing the same db session. This caused 'session is provisioning a new connection; concurrent operations are not permitted' errors for every series in the batch.

Fix by processing each batch sequentially instead of concurrently. BATCH_SIZE=10 still paces TMDB requests as intended.

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):