fix(test): add autouse fixture to reset worker_app.db module between tests

Add _reset_worker_app_db pytest fixture with autouse=True to prevent
mock pollution from leaking between tests. This fixes the flaky
test_batch_download_session_closed test that would fail when running
the full test suite due to worker_app.db being replaced with a
MagicMock from a previous test.

tests/unit/test_batch_download.py

@@ -13,6 +13,20 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+
+@pytest.fixture(autouse=True)
+def _reset_worker_app_db():
+    """Prevent mock pollution from leaking between tests."""
+    import importlib
+    import sys
+    # Save original module reference
+    original = sys.modules.get("worker_app.db")
+    yield
+    # Restore original module after each test
+    if original is not None:
+        sys.modules["worker_app.db"] = original
+
+
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