Likes and boosts arrive, count, notify, and can be taken back

- Like: a Favourite per account and post (unique), the post's counter, a
  Favourite notification for a local author; only on posts the liker
  could see (public and unlisted, or followers-only and direct when they
  were addressed).
- Announce: the original is always refetched from its own origin (or is
  ours), and must be public or unlisted. A boost of a local post counts
  and notifies; a boost by an account a persona follows is stored as a
  reblog row and fanned out to its followers' home timelines (as a reblog,
  where reblogs are wanted). A boost by a stranger of a stranger's post is
  ignored. FEP-1b12 group announces of activities wait for P4.
- Undo of a Like or an Announce reverses each of them; an Undo naming only
  an id is matched against follows, likes and boosts in turn.

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

PrivaPub.Tests/Federation/InboxScenarioTests.cs

@@ -53,7 +53,7 @@ namespace PrivaPub.Tests.Federation
 			_processor = new InboxProcessor(remote, new IActivityHandler[]
 			{
 				new FollowHandler(db, _local, remote, delivery),
-				new UndoHandler(db, _local, remote, delivery),
+				new UndoHandler(db, _local),
 				new CreateHandler(db, _local, remote, delivery, _blocks, new Fanout(db), new RemotePosts(db, _local, remote, _blocks, queue)),
 				new DeleteHandler(db, _local, remote, delivery),
 				new UpdateHandler(db, _local, remote)