Creator Partnership Read Proof Endpoint

One creator-submitted proof file, sent either inline or by link.

content_base64 is what the landing-page form sends: it holds the file in the browser, so it can post the bytes straight into the request. source_url is for the intake paths that only ever have a link — a Google Form's Apply Script hands over a Drive file, and re-downloading it in the browser just to re-upload it would be silly. Either way the file ends up on our own bucket, so nothing downstream depends on the original link staying reachable.