Register a webhook endpoint for release events

Body: { url, description?, event_types? }. Supported events: release.status_changed (every aggregate status transition, and every store that goes live or is taken down while the aggregate stays put, such as an added store finishing delivery or a partial takedown completing; storeStatuses carries the per-store status text) and release.approved (fires once when the release passes distributor approval and goes live; the payload carries the UPC and every track ISRC). 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.