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/database/connection.py

@@ -264,3 +264,35 @@ def get_sync_session() -> Session:
         )
     
     return _sync_session_factory()
+
+
+def get_async_session_factory() -> AsyncSession:
+    """Get a new async database session (factory function).
+    
+    Creates a new session instance for use in repository patterns.
+    The caller is responsible for committing/rolling back and closing.
+    
+    Returns:
+        AsyncSession: New database session for async operations
+    
+    Raises:
+        RuntimeError: If database is not initialized
+    
+    Example:
+        session = get_async_session_factory()
+        try:
+            result = await session.execute(select(AnimeSeries))
+            await session.commit()
+            return result.scalars().all()
+        except Exception:
+            await session.rollback()
+            raise
+        finally:
+            await session.close()
+    """
+    if _session_factory is None:
+        raise RuntimeError(
+            "Database not initialized. Call init_db() first."
+        )
+    
+    return _session_factory()