Posts reach their audience, edits follow them, deletes leave a tombstone

OutboxPublisher works out who a post goes to, Mastodon's way: followers'
shared inboxes for public, unlisted and followers-only; every mentioned
remote account's own inbox; the recipients only for direct; the parent's
author for a reply; a community's followers for a post in it; nobody for
a circle. Creates, updates and deletes all use it.

- Local posts take a visibility (public, unlisted, followers-only) and a
  spoiler text next to the content-warning flag.
- /clientapi/post/update keeps the previous version as a revision,
  re-renders, and sends Update{Note} with `updated` to the same to/cc.
- Deleting is now soft: the content, title, spoiler, media and revisions
  are cleared, timeline entries removed, the parent's reply count goes
  down, Delete goes to the stored audience, and the object answers 410
  with a Tombstone instead of 404.
- Editing a persona's profile sends Update{Person} to its followers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub.Tests/Support/RemoteActor.cs

@@ -24,6 +24,7 @@ namespace PrivaPub.Tests.Support
 		public string Name { get; }
 		public string Id { get; }
 		public string KeyId => Id + "#main-key";
+		public string SharedInbox => Id.Split("/users/")[0] + "/inbox";
 
 		public JsonObject Document() => new()
 		{