Get a short-lived download link and a preview for a file you own

The JSON form of GET /v1/files/{uuid}/download, built for agent callers that cannot follow an authenticated URL or show a binary body. No credits are charged and the file stays private: its visibility is not changed.

• download_url is a presigned, unauthenticated URL for the ORIGINAL file, valid for 15 minutes (expires_at). It is a credential for that time: anyone holding it can read the file. Do not log it or pass it on. It is null for a file stored only on the server disk (no expiring link can be issued); link_unavailable_reason then says why.
• preview is set for raster images only: a reduced JPEG (at most 1024 px on the long edge and about 350 KB), base64-encoded. It is never the original bytes. null for other files, SVG, or an image that cannot be decoded.

Authorization is the same two-layer check as the download route: files:read at the gate, then the read scope of the file's own container. A denial is a 404, never a 403.