Set Auto Apply

Turn automatic application on or off for a contract, or set its recurring amount.

A reason is required for the same reason /override needs one: this changes how much a client is charged, and the row has to say who decided that and why.

The listing is returned with include_contract_id forced, so the contract just changed comes back even when the browse rules would not have offered it — the lesson from the override endpoint, where a successful write could look like nothing happened.