Fix: Remove episodes from missing list on download/rescan

- Update _update_series_in_db to sync missing episodes bidirectionally
- Add delete_by_series_and_episode method to EpisodeService
- Remove downloaded episodes from DB after successful download
- Clear anime service cache when episodes are removed
- Fix tests to use 'message' instead of 'detail' in API responses
- Mock DB operations in rescan tests

src/server/database/service.py

@@ -393,6 +393,51 @@ class EpisodeService:
         )
         return result.rowcount > 0
 
+    @staticmethod
+    async def delete_by_series_and_episode(
+        db: AsyncSession,
+        series_key: str,
+        season: int,
+        episode_number: int,
+    ) -> bool:
+        """Delete episode by series key, season, and episode number.
+        
+        Used to remove episodes from the missing list when they are
+        downloaded successfully.
+        
+        Args:
+            db: Database session
+            series_key: Unique provider key for the series
+            season: Season number
+            episode_number: Episode number within season
+        
+        Returns:
+            True if deleted, False if not found
+        """
+        # First get the series by key
+        series = await AnimeSeriesService.get_by_key(db, series_key)
+        if not series:
+            logger.warning(
+                f"Series not found for key: {series_key}"
+            )
+            return False
+        
+        # Then delete the episode
+        result = await db.execute(
+            delete(Episode).where(
+                Episode.series_id == series.id,
+                Episode.season == season,
+                Episode.episode_number == episode_number,
+            )
+        )
+        deleted = result.rowcount > 0
+        if deleted:
+            logger.info(
+                f"Removed episode from missing list: "
+                f"{series_key} S{season:02d}E{episode_number:02d}"
+            )
+        return deleted
+
 
 # ============================================================================
 # Download Queue Service