- Add UnitOfWork context manager in domain/unit_of_work.py with commit/rollback/flush API and auto-rollback on exception
- Remove all db.commit() from test_workflow_service (8 calls), notification_service (4 calls), status_service (1 call)
- Services now only stage changes via db.add/db.flush; caller owns the transaction boundary
- Update routers/tests.py: wrap 9 workflow endpoints in UnitOfWork context managers
- Update routers/notifications.py: wrap mark_as_read and mark_all_as_read in UnitOfWork