Get short-lived presigned download links for a finished export

Any org member. Returns one presigned GET per produced object (data + schema sidecar), signed by a read-only session scoped to this job's prefix alone. Each link expires in 15 minutes — re-request them rather than storing them. Only a succeeded job has links. The export itself is deleted from object storage 7 days after it is produced; see the transfer/limits endpoint for this deployment's exact values.