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

src/server/fastapi_app.py

@@ -112,6 +112,16 @@ async def lifespan(app: FastAPI):
         # Subscribe to progress events
         progress_service.subscribe("progress_updated", progress_event_handler)
 
+        # Initialize download service and restore queue from database
+        try:
+            from src.server.utils.dependencies import get_download_service
+            download_service = get_download_service()
+            await download_service.initialize()
+            logger.info("Download service initialized and queue restored")
+        except Exception as e:
+            logger.warning("Failed to initialize download service: %s", e)
+            # Continue startup - download service can be initialized later
+
         logger.info("FastAPI application started successfully")
         logger.info("Server running on http://127.0.0.1:8000")
         logger.info(