Download an event's payload

The exact bytes a callback carried, as an opaque attachment whatever the sender declared, so third-party content is never rendered on the API origin. The recorded media type is in X-Ironfang-Payload-Media-Type and the SHA-256 in X-Ironfang-Payload-SHA256; the same digest is in the event's data. Events without a payload (anything but a received callback) are not found.