Fetch the signer's copy of the document

Record access for the signer (ATL-232 / ATL-285 / ATL-286): returns the PDF of the document as it stands at this signer's turn, so they can read it outside the viewer, keep a copy, or hand it to counsel before signing. Matches the viewer: earlier signers' completed values are stamped in, and blocks still awaiting a signature (including this signer's own) are marked, so the copy is never a blank original. Stamped from the signer's own view, so a later party's values can never appear. Served same-origin rather than redirected to storage: a browser fetch that followed a 302 to private Vercel Blob CORS-preflights that host, and Blob OPTIONS is not 2xx, so Download failed on staging. Same content gate as GET /envelope: a participant whose turn has not arrived, a terminal envelope, and a missing renderable document are all 404. Being a plain URL (the token may ride in the token query param), it is safe to open in a new tab.