fix(tests): dispose the global app engine at session teardown

backend/tests/conftest.py

@@ -2,6 +2,7 @@
 from pathlib import Path
 
 import pytest
+import pytest_asyncio
 from cryptography.fernet import Fernet
 
 os.environ.setdefault("DATABASE_URL", "sqlite+aiosqlite:///:memory:")
@@ -12,6 +13,24 @@
 os.environ.setdefault("AUTH_COOKIE_ENABLED", "false")
 
 
+@pytest_asyncio.fixture(scope="session", autouse=True)
+async def _dispose_app_engine():
+    """Dispose the global app engine before the session event loop closes.
+
+    Tests that exercise the real ``app.models.base.async_session_factory`` open
+    aiosqlite connections on the module-level ``engine`` that nothing else tears
+    down. With a session-scoped loop those connections' worker threads outlive
+    individual tests; when the loop finally closes, the daemon thread calls
+    ``call_soon_threadsafe`` on it and pytest surfaces a
+    ``PytestUnhandledThreadExceptionWarning: RuntimeError: Event loop is closed``.
+    Disposing here closes those connections while the loop is still running.
+    """
+    yield
+    from app.models import base as base_mod
+
+    await base_mod.engine.dispose()
+
+
 def pytest_collection_modifyitems(items: list[pytest.Item]) -> None:
     """Auto-apply 'unit' or 'integration' markers based on test file location."""
     for item in items: