Thread created

A new top-level thread was started in a newsletter's space (thread_level = 0).

data.visibility decides where it appears: public puts it on the global feed and only team members may set it, subscribers keeps it inside the newsletter's own space. paid exists in the enum and is not in use yet.

A thread created public fires thread.published from the same write, so a consumer that only cares about the feed can subscribe to that topic alone and ignore this one.

Delivered as a single HTTPS POST to the consumer's registered endpoint, with the message as the JSON request body. Respond 2xx to acknowledge. Anything else, or a timeout, is retried with backoff, so acknowledge fast and do the work afterwards.