Cancel Job

Cancel an in-flight job.

Routes a CancelRequest through the scheduler (the single writer of jobs / computation_node) and awaits the outcome — never writes the DB directly. The scheduler flips the job + its non-terminal nodes to cancelled, releases the concurrency lease, and interrupts in-flight worker executions.

Idempotent: cancelling an already-cancelled job is a 200 no-op; a completed / failed job is a 409 (you can't cancel a finished job); an unknown job — or one belonging to another org — is a 404.