The async spine for the heavy chain (sources/separate/loops): the API enqueues
and returns a job_id; a worker runs it out-of-band so the API never blocks.
Built to the four decisions taken with PLN:

- HYBRID sync/async: the 11 cheap analyses stay synchronous (cache-backed);
  only heavy work becomes a job. The line is wall-clock cost, not endpoint kind.
- PER-ENGINE submit + SHARED poll: engines return 202 {job_id} (wired in
  #29/#30/#31); GET /jobs/{id} polls, DELETE /jobs/{id} cancels. Typed JobStatus
  response_model so the generated client gets audio.getJob()/cancelJob().
- SINGLE serialized worker: one job at a time (erable is 2 GB / no GPU; two
  demucs runs would OOM). The atomic claim is still race-safe for N workers.
- Identity = gateway tenant; a job is visible only to its owner (cross-tenant
  poll/cancel → 404, not 403, so existence doesn't leak).

Pieces:
- db.py: jobs table (status/priority/progress/result/attempts/webhook) + a
  claim-ordered index.
- jobs.py: enqueue (incl. born-done idempotent fast path when the cache already
  has the result), atomic claim (candidate → guarded UPDATE WHERE status=
  'pending'; rowcount-0 retry; WAL serializes writers so no double-claim),
  progress/finish/fail-with-requeue-under-cap, crash recovery (running→pending
  on boot), cooperative cancel + is_cancelled checkpoints.
- worker.py: serialized loop — recover → claim → dispatch by type → finish/fail
  → optional webhook; SIGTERM-graceful; imports engine handler modules (none yet,
  idles politely); engines register via @jobs.handler.
- deploy/douanier-worker.service: systemd --user unit (linger) — the durable run
  path (harness/nohup jobs die on teardown). Prod container-vs-host wiring is the
  paved-road call (SRE/#34).

VALIDATION: tests/test_jobs.py — submit→claim→done; born-done fast path;
finish/requeue-to-cap; crash recovery; NO double-claim across 6 threads × 25
jobs (each claimed exactly once); worker dispatch to done; missing-handler error;
exception→requeue; cancel mid-flight; HTTP poll + ownership 404 + idempotent
cancel. 11 new tests, full suite 67→78 green. OpenAPI snapshot refreshed (15
ops) + @nech/api regenerated (getJob/cancelJob); noImplicitAny relaxed for the
100%-generated client (typescript-fetch's camel/snake guard trips TS7053).