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/activities/add.rs

@@ -54,7 +54,14 @@ impl Activity for AddActivity {
         if check_guards(&self.id, self.actor.inner(), data).await? {
             return Ok(());
         }
-        let ap_id = self.id.clone();
+        // Use the object's own id as the stable AP identifier, falling back to
+        // the activity id only if the object has no id field.
+        let ap_id = self
+            .object
+            .get("id")
+            .and_then(|v| v.as_str())
+            .and_then(|s| Url::parse(s).ok())
+            .unwrap_or_else(|| self.id.clone());
         let actor_url = self.actor.inner().clone();
         data.object_handler
             .on_create(&ap_id, &actor_url, self.object)