Webhook payloads

Endpoint shape implemented by external webhook receivers. Return any 2xx response after accepting an event. - Delivery is at least once. Deduplicate on the `webhook-id` header, which equals the payload `id` and stays the same on every retry of an event. The body is identical on every retry. - Delivery order isn't guaranteed, even within one conversation. For `message.updated`, keep the snapshot with the highest `message.revision`. - Each attempt times out after 15 seconds. Any 2xx response acknowledges the event. Redirects aren't followed. - Timeouts, connection errors, 3xx, 408, 429 and 5xx responses are retried about 30 seconds, 5 minutes and 30 minutes after the first attempt, for 4 attempts in total. Retry times include up to 20% random jitter, and the last retry happens within 30 minutes of the first attempt. - After a 429, a `Retry-After` value in seconds makes the next attempt wait at least that long. If that is later than 30 minutes after the first attempt, the event isn't retried. - 410 and any other 4xx response stop retries for that event. - An event that isn't delivered after its last attempt isn't sent again. Flownally never disables the endpoint automatically. - `webhook-timestamp` and `webhook-signature` are generated again for each attempt. - `GET /webhooks/attempts` lists delivery attempts from the last 7 days. - Events are delivered only while the webhook is enabled and verified. Changing `url` clears `verified`; deliveries resume after `POST /webhooks:verify` succeeds for the saved URL. Events from the time in between aren't delivered later.