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

tests/frontend/test_existing_ui_integration.py

@@ -201,7 +201,7 @@ class TestFrontendAnimeAPI:
     
     async def test_rescan_anime(self, authenticated_client):
         """Test POST /api/anime/rescan triggers rescan with events."""
-        from unittest.mock import MagicMock
+        from unittest.mock import MagicMock, patch
 
         from src.server.services.progress_service import ProgressService
         from src.server.utils.dependencies import get_anime_service
@@ -210,7 +210,7 @@ class TestFrontendAnimeAPI:
         mock_series_app = MagicMock()
         mock_series_app.directory_to_search = "/tmp/test"
         mock_series_app.series_list = []
-        mock_series_app.rescan = AsyncMock()
+        mock_series_app.rescan = AsyncMock(return_value=[])
         mock_series_app.download_status = None
         mock_series_app.scan_status = None
         
@@ -232,7 +232,16 @@ class TestFrontendAnimeAPI:
         app.dependency_overrides[get_anime_service] = lambda: anime_service
         
         try:
-            response = await authenticated_client.post("/api/anime/rescan")
+            # Mock database operations called during rescan
+            with patch.object(
+                anime_service, '_save_scan_results_to_db', new_callable=AsyncMock
+            ):
+                with patch.object(
+                    anime_service, '_load_series_from_db', new_callable=AsyncMock
+                ):
+                    response = await authenticated_client.post(
+                        "/api/anime/rescan"
+                    )
             
             assert response.status_code == 200
             data = response.json()
@@ -448,7 +457,7 @@ class TestFrontendJavaScriptIntegration:
         assert response.status_code in [200, 400]
         if response.status_code == 400:
             # Verify error message indicates empty queue
-            assert "No pending downloads" in response.json()["detail"]
+            assert "No pending downloads" in response.json()["message"]
         
         # Test pause - always succeeds even if nothing is processing
         response = await authenticated_client.post("/api/queue/pause")