Resolve a public download token

Resolves a public download token to minimal file information for the download page. No authentication required. Returns fileName, mediaType, status, the link mode, and (for image assets) a `previewKeyPath` so the page can render an inline preview.

curl -X GET "https://example.com/public-download/Ab3dEf9HiJ"

{  "token": "string",  "fileName": "my-video.mp4",  "mediaType": "video",  "status": "active",  "mode": "embed-download",  "mimeType": "image/png",  "previewKeyPath": "string"}