A pre-signed URL that allows direct upload to Google Cloud Storage (GCS). This URL is time-limited and includes authentication signature. Use this URL with PUT method to upload the file directly to GCS without going through the API server. Example: "https://storage.googleapis.com/bucket/path?signature=...".

The time in seconds until the upload URL expires. Default is 1800 seconds (30 minutes). After expiry, the URL becomes invalid and a new one must be requested. The client must complete the upload before this time. Example: 1800.