Fix deck test failures - SQLite RETURNING clause and async serialization

- Fixed conftest.py: Use execution_options for implicit_returning (SQLAlchemy 2.0)
- Fixed models.py: Changed UserDeck.updated_at onupdate from func.now() to datetime.now
- All 5 deck tests now pass
- Full suite: 26 passed, 2 failed (pre-existing game model issues)

backend/tests/conftest.py

@@ -16,18 +16,12 @@ from app.models.models import User
 # Use in-memory SQLite for testing
 TEST_DATABASE_URL = "sqlite+aiosqlite:///:memory:"
 
-# Create engine with RETURNING disabled
+# Create engine with RETURNING disabled for SQLite
 test_engine = create_async_engine(
     TEST_DATABASE_URL,
     connect_args={"check_same_thread": False},
     poolclass=StaticPool,
-    execution_options={
-        "insertmanyvalues_page_size": 0,
-        "implicit_returning": False,
-        "postgresql_force_nulls": False,
-    },
-    # Disable RETURNING for SQLite
-    echo=False,
+    execution_options={"implicit_returning": False},
 )
 
 TestAsyncSession = async_sessionmaker(test_engine, class_=AsyncSession, expire_on_commit=False)