Create webhook endpoint

Register an https URL that hears about the work this connection can read: post.scheduled, post.published, post.failed, generation.finished and text_generation.finished. Each event is a POST of JSON with its id, type, created_at and data (object, id, brand_id, status); read the object through its GET. The SocialFaktory-Signature header carries t=<unix seconds>,v1=<hex HMAC-SHA256 of "<t>.<raw body>" under the secret>. Reject a t older than 5 minutes, and dedupe on the event id, since a retry resends the same event. The secret is shown in this answer only, and again to a retry of this call under the same idempotency key. A connection holds at most 5 endpoints, and one whose deliveries fail 10 events in a row turns disabled.