Migrate download queue from JSON to SQLite database

- Created QueueRepository adapter in src/server/services/queue_repository.py
- Refactored DownloadService to use repository pattern instead of JSON
- Updated application startup to initialize download service from database
- Updated all test fixtures to use MockQueueRepository
- All 1104 tests passing

tests/integration/test_websocket_integration.py

@@ -69,16 +69,17 @@ async def anime_service(mock_series_app, progress_service):
 
 @pytest.fixture
 async def download_service(anime_service, progress_service, tmp_path):
-    """Create a DownloadService with dependencies.
+    """Create a DownloadService with mock repository for testing.
     
-    Uses tmp_path to ensure each test has isolated queue storage.
+    Uses mock repository to ensure each test has isolated queue storage.
     """
-    import uuid
-    persistence_path = tmp_path / f"test_queue_{uuid.uuid4()}.json"
+    from tests.unit.test_download_service import MockQueueRepository
+    
+    mock_repo = MockQueueRepository()
     service = DownloadService(
         anime_service=anime_service,
         progress_service=progress_service,
-        persistence_path=str(persistence_path),
+        queue_repository=mock_repo,
     )
     yield service, progress_service
     await service.stop()