Retrieve a pre-signed URL for the specified embed event file.

The event's data snapshot is persisted to durable storage shortly after the event is recorded. Until that completes, this endpoint responds with 409 and code snapshot_not_ready; retry after the number of seconds given in the Retry-After header. If the snapshot could not be persisted (snapshot_failed) or was discarded because its list no longer exists (snapshot_discarded), this endpoint responds with 410 and the file is permanently unavailable.