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/performance/test_download_stress.py

@@ -28,12 +28,13 @@ class TestDownloadQueueStress:
 
     @pytest.fixture
     def download_service(self, mock_anime_service, tmp_path):
-        """Create download service with mock."""
-        persistence_path = str(tmp_path / "test_queue.json")
+        """Create download service with mock repository."""
+        from tests.unit.test_download_service import MockQueueRepository
+        mock_repo = MockQueueRepository()
         service = DownloadService(
             anime_service=mock_anime_service,
             max_retries=3,
-            persistence_path=persistence_path,
+            queue_repository=mock_repo,
         )
         return service
 
@@ -176,12 +177,13 @@ class TestDownloadMemoryUsage:
 
     @pytest.fixture
     def download_service(self, mock_anime_service, tmp_path):
-        """Create download service with mock."""
-        persistence_path = str(tmp_path / "test_queue.json")
+        """Create download service with mock repository."""
+        from tests.unit.test_download_service import MockQueueRepository
+        mock_repo = MockQueueRepository()
         service = DownloadService(
             anime_service=mock_anime_service,
             max_retries=3,
-            persistence_path=persistence_path,
+            queue_repository=mock_repo,
         )
         return service
 
@@ -232,12 +234,13 @@ class TestDownloadConcurrency:
 
     @pytest.fixture
     def download_service(self, mock_anime_service, tmp_path):
-        """Create download service with mock."""
-        persistence_path = str(tmp_path / "test_queue.json")
+        """Create download service with mock repository."""
+        from tests.unit.test_download_service import MockQueueRepository
+        mock_repo = MockQueueRepository()
         service = DownloadService(
             anime_service=mock_anime_service,
             max_retries=3,
-            persistence_path=persistence_path,
+            queue_repository=mock_repo,
         )
         return service
 
@@ -321,11 +324,12 @@ class TestDownloadErrorHandling:
         self, mock_failing_anime_service, tmp_path
     ):
         """Create download service with failing mock."""
-        persistence_path = str(tmp_path / "test_queue.json")
+        from tests.unit.test_download_service import MockQueueRepository
+        mock_repo = MockQueueRepository()
         service = DownloadService(
             anime_service=mock_failing_anime_service,
             max_retries=3,
-            persistence_path=persistence_path,
+            queue_repository=mock_repo,
         )
         return service
 
@@ -338,12 +342,13 @@ class TestDownloadErrorHandling:
 
     @pytest.fixture
     def download_service(self, mock_anime_service, tmp_path):
-        """Create download service with mock."""
-        persistence_path = str(tmp_path / "test_queue.json")
+        """Create download service with mock repository."""
+        from tests.unit.test_download_service import MockQueueRepository
+        mock_repo = MockQueueRepository()
         service = DownloadService(
             anime_service=mock_anime_service,
             max_retries=3,
-            persistence_path=persistence_path,
+            queue_repository=mock_repo,
         )
         return service