fix: AP protocol correctness gaps

Undo(Announce): now removes announce record from ActorRepository and
  calls ApObjectHandler::on_announce_removed (default no-op, override
  to decrement boost counts). Announce counts no longer drift.

Undo(Block): now logged at info level instead of silently ignored.
  No automatic relationship restoration (spec doesn't require it).

AddActivity: now uses object["id"] as the stable ap_id (same as
  CreateActivity), falling back to activity id only if object has no
  id field. Fixes keying watchlist/collection items by the wrong id.

Featured collection: GET /users/{id}/featured now served by the router.
  ApContentReader::get_featured_objects() has a default empty-list impl
  — override to expose pinned posts without any breaking changes.

src/lib.rs

@@ -4,6 +4,7 @@ pub mod actors;
 pub mod content;
 pub mod data;
 pub mod error;
+pub mod featured_handler;
 pub mod federation;
 pub mod followers_handler;
 pub mod inbox;