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/featured_handler.rs

@@ -0,0 +1,42 @@
+use activitypub_federation::{axum::json::FederationJson, config::Data};
+use axum::extract::Path;
+use serde_json::json;
+
+use crate::data::FederationData;
+use crate::error::Error;
+use crate::urls::AP_CONTEXT;
+
+/// Serves the `featured` (pinned posts) `OrderedCollection` for a local user.
+///
+/// Remote servers follow the `featured` link from the actor JSON and expect
+/// an `OrderedCollection` whose `orderedItems` are the AP URLs of pinned objects.
+/// The handler calls [`ApContentReader::get_featured_objects`] — override that
+/// method to expose your pinned posts.
+pub async fn featured_handler(
+    Path(user_id_str): Path<String>,
+    data: Data<FederationData>,
+) -> Result<FederationJson<serde_json::Value>, Error> {
+    let user_id = uuid::Uuid::parse_str(&user_id_str)
+        .map_err(|_| Error::not_found(anyhow::anyhow!("user not found")))?;
+
+    data.user_repo
+        .find_by_id(user_id)
+        .await
+        .map_err(Error::from)?
+        .ok_or_else(|| Error::not_found(anyhow::anyhow!("user not found")))?;
+
+    let featured_url = format!("{}/users/{}/featured", data.base_url, user_id_str);
+    let items = data
+        .content_reader
+        .get_featured_objects(user_id)
+        .await
+        .map_err(|e| Error::from(anyhow::anyhow!("{}", e)))?;
+
+    Ok(FederationJson(json!({
+        "@context": AP_CONTEXT,
+        "type": "OrderedCollection",
+        "id": featured_url,
+        "totalItems": items.len(),
+        "orderedItems": items.iter().map(|u| u.as_str()).collect::<Vec<_>>(),
+    })))
+}