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

PrivaPub/Federation/Inbox/InboxReceiver.cs

@@ -235,8 +235,10 @@ namespace PrivaPub.Federation.Inbox
 					break;
 				case "Undo" when inner is JsonObject && Id(inner["actor"]) != actorUri:
 					return new(StatusCodes.Status400BadRequest, "an actor can only undo its own activities", Reason: "undo-foreign");
+				// (attributed to another account of the actor's own server, as Mobilizon's organiser creates a group's
+				// event: the handlers read it from that server, Origin.Same)
 				case "Create" or "Update" when inner is JsonObject && Origin.Same(Id(inner), actorUri)
-					&& inner["attributedTo"] != default && Id(inner["attributedTo"]) != actorUri
+					&& inner["attributedTo"] != default && !Origin.Same(Id(inner["attributedTo"]), actorUri)
 					&& Value(inner, "type") is not ("Person" or "Service" or "Application" or "Group" or "Organization"):
 					return new(StatusCodes.Status400BadRequest, "the object is not attributed to the actor", Reason: "misattributed");
 			}