Get one item's payload

Returns a single finished item's payload verbatim (not wrapped), capped at 512 KiB. Use it to fetch one URL's output without paging the whole result set. Status meanings: 200 payload; 409 either "not finished yet, poll the job and ask again" or "finished as failed/cancelled/skipped, will never have content" (read detail; for failures, POST .../retry); 404 seq is beyond the job's item count; 503 the deployment has no per-item payload to serve. Note: the worker currently writes one stitched file per job and no per-item reference, so this endpoint answers 503 for succeeded items in practice — read result.content from the results endpoint instead. Retention is not checked here.