An organiser's edits to a group's event follow its server

Mobilizon's organiser sends the Create, Update and Delete of an event attributed to the group, which announces the
Event itself. PrivaPub refused the organiser's activities as misattributed (400) and kept the event through the
group's Announce, so an edit was lost and a deletion left the event in place. An object attributed to another account
of the actor's own server is now that server's to vouch for: created or edited as the server has it, under the account
it is attributed to, and deleted once the server answers 404 or 410. Attributed to an account elsewhere, it is still
refused. Checked against Mobilizon 5.2.4 in the pasture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

FEDERATION.md

@@ -225,6 +225,10 @@ Posts with a location (shown to nearby users of this server) never leave the ser
 - **Origins.** An actor document is accepted only from the address it names as its `id`. A key only if its actor lists
   it with `owner` set to the actor and on the actor's origin. An activity's `id`, and any object it creates, updates or
   deletes, must be on its actor's origin. An embedded object from another origin is fetched from that origin.
+- **Another account of the same server.** An object attributed to another account of the actor's own server (Mobilizon's
+  organiser creates, edits and deletes the group's event) is that server's to vouch for: a creation or an edit is
+  taken as the server has the object, a deletion once it answers 404 or 410. Attributed to an account elsewhere, it is
+  refused (400).
 - **Forwarded activities** (ActivityPub's inbox forwarding: a thread's server passing on the replies in it, signed with
   its own key) are answered 202 and believed only as far as their origin vouches for them: a created or edited post
   is read again from its author's server, a deletion of a public or unlisted post is applied once that server answers