Fix async_client fixture to patch engine with in-memory DB

The fixture accepted in_memory_engine but never actually patched
database.engine or api.app.engine, causing tests to hit the real
SQLite path which fails in CI where data/ doesn't exist.

tests/conftest.py

@@ -165,25 +165,28 @@ async def async_client(
     in_memory_engine: Engine, mock_user: User
 ) -> AsyncGenerator[AsyncClient, None]:
     """Create an AsyncClient for API testing with mock auth."""
-    from api.app import app
+    import database
+    import api.app as api_app
     from api.auth import get_current_user
 
+    app = api_app.app
+
     # Override dependencies
     app.dependency_overrides[get_current_user] = lambda: mock_user
 
-    # Patch the engine used by the repository
-    original_engine = None
-    try:
-        from database import engine as db_engine
-        original_engine = db_engine
-    except Exception:
-        pass
+    # Patch the engine so the API uses the in-memory database
+    original_db_engine = database.engine
+    original_app_engine = api_app.engine
+    database.engine = in_memory_engine
+    api_app.engine = in_memory_engine
 
     transport = ASGITransport(app=app)
     async with AsyncClient(transport=transport, base_url="http://test") as client:
         yield client
 
-    # Clean up dependency overrides
+    # Restore original engines and clean up
+    database.engine = original_db_engine
+    api_app.engine = original_app_engine
     app.dependency_overrides.clear()