Download a binary file from an actor's output (e.g. a captured image)

Stream one blob a binary actor produced (the image-fetch actor's output).

A dataset row's download_url points here. Requires the same bearer token as the rest of the API and only serves the caller's own runs — an unknown or other-user run is a 404, so run ids cannot be probed. Files expire 24h after the run finishes; a missing or expired file is file_not_found.