Outbound event payload

Zentag POSTs this JSON body to your registered HTTPS endpoint whenever a
subscribed event occurs. Discriminate on type (see enum below).

Event types: clip.created, clip.ready, clip.failed,
highlight.ready, highlight.failed, publish.succeeded,
publish.failed, stream.started, stream.ended.

Verify the Zentag-Signature header (t=<unix>,v1=<hmac-sha256>) before
processing. Return 2xx to acknowledge; non-2xx responses are retried
with exponential backoff.