Delete a webhook endpoint

Register a webhook so long-running jobs (render/compose/media) notify you instead of blocking on wait_job. Body: `url` (https, must pass SSRF checks) + `events` (see the WebhookEvent enum). DELIVERY: each event is a POST with body `{ event, timestamp, projectId?, data }` and Standard-Webhooks headers `webhook-id`, `webhook-timestamp`, `webhook-signature: v1,<base64>` (+ `x-cueframe-event`). The `secret` (`whsec_<base64>`) is returned ONCE here — verify the raw body with `new Webhook(secret).verify(rawBody, headers)`. ACTIVATION (synchronous, no separate step): on create, CueFrame POSTs a signed `webhook.verify` challenge; your endpoint must reply 2xx echoing `data.token` (raw body === token, or JSON `{token}`/`{challenge}`). On success the response is `status:"active"`; otherwise `status:"pending"` with `verification.reason` and the webhook receives NO events until it verifies (recover with POST /webhooks/:id/verify — no delete/recreate). Full contract + per-event `data` shapes: GET /webhooks/events.