Thread published to the feed

A thread's visibility became public, which is what puts it on Commune's global feed. This is the only way community content leaves one newsletter's space and reaches everyone, so it is the topic to watch for anything that mirrors, syndicates or moderates the feed.

Two paths reach public and both publish here, told apart by data.source:

• visibility_changed: a team member featured a thread that already existed. published_by and published_at say who and when.
• created_public: the thread was born public, which a team member may do when starting a top-level thread. That write fires thread.created and this topic together, and carries no published_by.

Only an owner, admin or editor can reach public, and only on a top-level thread: featuring anything with a non-zero thread_level is refused, and replies inherit their thread's placement.

Going back to subscribers is a real change and has no topic. A consumer mirroring the feed should reconcile against GET /threads/{thread} rather than assume a thread it saw here is still public.

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.