Submit an image generation task

Optional. Public https URL with a domain-name host that receives one POST with the task response (same shape as GET /tasks/{id}) when the task reaches completed or failed. Rejected at submission: non-https schemes, IP-address hosts (IPv4 or IPv6, any notation), any port other than 443, URLs with credentials, reapi.ai hosts. Names that resolve to private, loopback, link-local or cloud-metadata addresses fail at delivery (the task itself still runs). Delivery is at-least-once (up to 3 attempts over ~5 minutes); deduplicate on the body id (the task id). reAPI sends no credentials and does not sign calls: treat a call as a signal and confirm it with an authenticated GET /tasks/{id}, or optionally embed a secret of your own in the URL. The webhook is a notification, not the only copy of the result: if delivery fails, GET /tasks/{id} still returns the task at any time (GET /tasks/{id}?include=webhook shows the delivery outcome).