Commune Webhooks API

The events Commune pushes to a consumer, rather than the resources aconsumer pulls. Commune publishes state changes on 23 topics, eachdelivered as one HTTPS POST to an endpoint the consumer registered.Every message shares one envelope, so a consumer can route on `type` anddedupe on `id` without knowing anything about the specific event, and thesame values arrive as `Commune-Event-Type` and `Commune-Event-Id` headersso both can be read before the body is parsed.Delivery is at least once and unordered. A non-2xx response or a timeoutis retried with backoff, so a consumer has to treat `id` as the dedupe keyand tolerate replays. `occurred_at` is the ordering field, not arrivaltime.Two envelope fields say where a change came from rather than whatchanged. `actor` names the credential when the change was made throughthis API's write operations, and `idempotency_key` carries the key thatwrite was made under. Both are `null` for a change made anywhere else,which is most of them.**If your consumer writes, read `idempotency_key` before you act.** Awrite through this API publishes an event, and that event is delivered toevery endpoint registered for the newsletter, including yours. A consumerthat reacts to events by writing therefore receives the echo of its ownwrite, cannot tell it from a change somebody else made, and writes again.You already hold what breaks the loop: you generated the key you sent onthe write, so keep it and skip any event whose `idempotency_key` is one ofyours. `actor` is not the field for this. It names Commune's own id foryour credential, and no operation here tells you what that id is.Registering an endpoint happens in the delivery portal, which`POST /newsletters/{newsletter}/portal-session` mints a link into. Theendpoints already registered are readable at`GET /newsletters/{newsletter}/destinations`.What happened to a particular message is readable.`GET /newsletters/{newsletter}/delivery-attempts` lists every handoverCommune made, filterable by the `event_id` a consumer reads off its own`Commune-Event-Id` header, so "did that event reach me" is answerable fromboth sides of the same identifier.