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/Support/Harness.cs

@@ -45,7 +45,9 @@ namespace PrivaPub.Tests.Support
 				new FollowHandler(Db, Local, Remote, Delivery),
 				new AcceptHandler(Db, Local),
 				new RejectHandler(Db, Local),
-				new UndoHandler(Db, Local, Remote, Delivery),
+				new UndoHandler(Db, Local),
+				new LikeHandler(Db),
+				new AnnounceHandler(Db, Local, RemotePosts, Fanout),
 				new CreateHandler(Db, Local, Remote, Delivery, new NoBlocks(), Fanout, RemotePosts),
 				new DeleteHandler(Db, Local, Remote, Delivery),
 				new UpdateHandler(Db, Local, Remote)