Cancel multiple jobs

Cancel one or more jobs for the authenticated user in a single request.

State-agnostic: cancels both pending and running jobs (both transition to the cancelled state via the same mechanism as the single-job endpoint).

Idempotent per job: a job already in a terminal or cancelling state is a no-op and simply will not appear in the returned cancelled list.

Fail-fast on unknown IDs: if any provided job ID does not exist for this user, the request returns 404 and no jobs are cancelled. This surfaces bad IDs to the caller rather than silently dropping them.

This is the canonical batch-cancel endpoint. The delete operation on POST /api/queue is deprecated in favour of this.