feat(fediverse): federate post edits as Update (Mastodon refreshes cached copy)
Editing an already-published, federated post now sends Update(Note) to followers
(with an 'updated' timestamp), so Mastodon refreshes the cached post instead of
keeping the stale original — e.g. fixes a post that federated before content
handling existed. New-publish still sends Create; fan_only never federates.
Co-Authored-By: Claude <noreply@…>