Prepare file uploads

Validates up to 10 files (1GB each), persists pending records, and returns a presigned PUT URL, required headers, and an expiry per file. PUT each file's bytes to its uploadUrl with the returned headers, then call the complete endpoint with the file ids. Target a folder with folderId or folderPath (mutually exclusive); folderPath creates missing folders along the way. Pending uploads that are never completed expire and are cleaned up automatically.