Register a webhook endpoint for release events

Body: { url, description?, event_types? }. Supported events: release.status_changed (every aggregate status transition), release.approved (fires once when the release passes distributor approval and goes live; the payload carries the UPC and every track ISRC), release.stores_added (an already-distributed release was queued for additional stores) and release.takedown_completed (stores confirmed a full or partial takedown). The last two have no aggregate status transition to ride on, so they only reach you as their own events. Every payload also carries artistProfiles with the Apple Music artist id and Spotify artist URI pinned to the release. New endpoints subscribe to all events unless event_types narrows the list. Endpoints must use https; up to 5 per account. The response includes the signing secret exactly once, so store it immediately.