Add filter for series with no downloaded episodes

- Added get_series_with_no_episodes() method to AnimeSeriesService
- Updated list_anime endpoint to support filter='no_episodes' parameter
- Added comprehensive unit tests for the new filtering functionality
- All tests passing successfully

src/server/database/service.py

@@ -251,6 +251,59 @@ class AnimeSeriesService:
             .limit(limit)
         )
         return list(result.scalars().all())
+    
+    @staticmethod
+    async def get_series_with_no_episodes(
+        db: AsyncSession,
+        limit: Optional[int] = None,
+        offset: int = 0,
+    ) -> List[AnimeSeries]:
+        """Get anime series that have no downloaded episodes in folder.
+        
+        Returns series where either:
+        - No episodes exist in the database, OR
+        - All episodes have is_downloaded=False
+        
+        Args:
+            db: Database session
+            limit: Optional limit for results
+            offset: Offset for pagination
+        
+        Returns:
+            List of AnimeSeries instances with no downloaded episodes
+        """
+        from sqlalchemy import func, or_
+        
+        # Subquery to count downloaded episodes per series
+        downloaded_count = (
+            select(
+                Episode.series_id,
+                func.count(Episode.id).label('downloaded_count')
+            )
+            .where(Episode.is_downloaded.is_(True))
+            .group_by(Episode.series_id)
+            .subquery()
+        )
+        
+        # Select series that either have no episodes or no downloaded episodes
+        query = (
+            select(AnimeSeries)
+            .outerjoin(downloaded_count, AnimeSeries.id == downloaded_count.c.series_id)
+            .where(
+                or_(
+                    downloaded_count.c.downloaded_count == None,
+                    downloaded_count.c.downloaded_count == 0
+                )
+            )
+            .order_by(AnimeSeries.name)
+            .offset(offset)
+        )
+        
+        if limit:
+            query = query.limit(limit)
+        
+        result = await db.execute(query)
+        return list(result.scalars().all())
 
 
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