Create several assets from one upload, asynchronously

Accepts several files plus a JSON form part naming the target and the base type, and answers 202 with a job handle before the assets are created. The work runs in the background: follow it with GET /api/v1/jobs/{jobId}/status, watch it with GET /api/v1/jobs/{jobId}/monitor, cancel it with POST /api/v1/jobs/{jobId}/cancel, and read the per-file outcome from the job's result. The submitting author is notified on any terminal state, so the outcome survives them navigating away.

One file failing does not abort the batch: every remaining file is still attempted, and each failure is reported by name with a machine-readable reason.