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/service/mod.rs

@@ -9,6 +9,7 @@ use crate::{
     actors::{DbActor, get_local_actor},
     content::{ApContentReader, ApObjectHandler},
     data::FederationData,
+    featured_handler::featured_handler,
     federation::ApFederationConfig,
     followers_handler::{followers_handler, following_handler},
     inbox::inbox_handler,
@@ -210,6 +211,7 @@ impl ActivityPubService {
             .route("/users/{id}/outbox", get(outbox_handler))
             .route("/users/{id}/followers", get(followers_handler))
             .route("/users/{id}/following", get(following_handler))
+            .route("/users/{id}/featured", get(featured_handler))
             .layer(self.federation_config.middleware())
     }