Add two cancel endpoints under the jobs namespace so a job can be
cancelled by id without the caller needing to know whether the job is
running or pending, or branching between /interrupt and /queue.
- POST /api/jobs/{job_id}/cancel cancels one job by id. Idempotent: an
already-finished or unknown id returns 200 {"cancelled": false} rather
than an error.
- POST /api/jobs/cancel takes {"job_ids": [...]} and cancels a batch.
Fail-fast: if any id is unknown the request returns 404 listing the
unknown ids and cancels nothing (no partial side effects).
Both are state-agnostic and map onto the existing queue mechanics: a
running job is interrupted (same path as /interrupt), a pending job is
dequeued (same path as /queue {"delete": [...]}). The cancel logic lives
in comfy_execution.jobs as pure, unit-tested helpers; the server handlers
are thin wrappers. openapi.yaml documents both routes.
