Request presigned upload targets (DEFAULT ingestion path)

Optional. Retry a write with the same key and you get the ORIGINAL response back instead of the work happening twice — so a request that timed out is safe to repeat. Use one key per logical operation (a UUID), and reuse it for every retry of that operation. Keys are scoped to your API key and honoured for 24 hours. A replayed response carries the Idempotent-Replayed: true header — the body is byte-identical to the original, so that is the only way to tell stored work from fresh work. Reusing a key with a different request body returns 409 idempotency_key_reuse; replaying while the first request is still running returns 409 idempotency_request_in_progress. An empty header, or one over 255 characters, is rejected with 400 invalid_idempotency_key — omit it entirely to opt out.