Create subscription
Register a watchlist webhook. RESTful alias of `POST /webhooks/subscribe` with identical behavior. Captain POSTs a `watchlist.match` body to your `callback_url`; see the `WatchlistMatchPayload` schema.
**Auth:** send `Authorization: Bearer <api_key>`. Include `X-Organization-ID` when your key is not already scoped to an organization. Subscriptions are visible only to the organization that created them.