Measure the cost of task grant status checks

Follow-up to #1029. Both behaviours fail closed, which is intended; this is about load and latency, not correctness.

- BREG `check_task_authority` calls the Casework status endpoint while holding the proposal row lock and a database connection, with a 5 s timeout. A slow Casework can hold both for up to 5 s per grant-bound write.
- Casework `task_status` does a live source read on every call, so each agent write costs a source round trip, and a source outage stops agent writes.

- [ ] Load test grant-bound BREG writes with Casework and the source slowed down, and record pool use and latency
- [ ] Based on the result, decide whether the status call moves outside the lock, or Casework reuses source context within the grant lifetime, or nothing changes

Start at the BREG check_task_authority entry point and the Casework task_status endpoint described in the issue. Run load tests with Casework and the source slowed down, recording connection-pool use and latency for grant-bound writes. Done means the results are documented and one of the three status-call options is selected.

Written by the indexing model from the issue text.