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/repository/actor.rs

@@ -30,5 +30,8 @@ pub trait ActorRepository: Send + Sync {
         actor_url: &str,
         announced_at: chrono::DateTime<chrono::Utc>,
     ) -> Result<()>;
+    /// Remove a boost record when a remote actor sends `Undo(Announce)`.
+    /// Implementations should match by `activity_id` and `actor_url`.
+    async fn remove_announce(&self, activity_id: &str, actor_url: &str) -> Result<()>;
     async fn count_announces(&self, object_url: &str) -> Result<usize>;
 }