* Make `userId` required on DB entities `AgentGraph`, `AgentGraphExecution`, and `AgentGraphExecutionSchedule`
* Add SQLite and Postgres migrations to make `userId` required and set `userId` to `3e53486c-cf57-477e-ba2a-cb02dc828e1a` on existing entries without `userId`
* Amend `create_graph` endpoint and `.data.graph`, `.data.execution` methods to handle required `user_id`
* Add `.data.user.DEFAULT_USER_ID` constant to replace hardcoded literals