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/unit/test_download_progress_websocket.py

@@ -102,27 +102,20 @@ async def anime_service(mock_series_app, progress_service):
 
 @pytest.fixture
 async def download_service(anime_service, progress_service):
-    """Create a DownloadService with dependencies."""
-    import os
-    persistence_path = "/tmp/test_download_progress_queue.json"
+    """Create a DownloadService with mock repository for testing."""
+    from tests.unit.test_download_service import MockQueueRepository
     
-    # Remove any existing queue file
-    if os.path.exists(persistence_path):
-        os.remove(persistence_path)
+    mock_repo = MockQueueRepository()
     
     service = DownloadService(
         anime_service=anime_service,
         progress_service=progress_service,
-        persistence_path=persistence_path,
+        queue_repository=mock_repo,
     )
     
     yield service, progress_service
     
     await service.stop()
-    
-    # Clean up after test
-    if os.path.exists(persistence_path):
-        os.remove(persistence_path)
 
 
 class TestDownloadProgressWebSocket: