fix: AP bugs — backfill mapping, review activity type, also_known_as parse

- BackfillRequested now maps to BackfillFollower domain event (not FollowAccepted);
  worker calls run_backfill_for_follower to send LOCAL content to new follower inbox,
  instead of incorrectly trying to import from an inbox URL as if it were an outbox
- reviews broadcast as Create activity instead of Add (semantically correct)
- also_known_as JSON parse failure logs warning + preserves raw string as single-element
  vec instead of silently returning empty

crates/adapters/activitypub/src/event_handler.rs

@@ -123,7 +123,7 @@ impl ActivityPubEventHandler {
         let json = serde_json::to_value(obj)?;
 
         self.ap_service
-            .broadcast_add_to_followers(user_id.value(), ap_id, json)
+            .broadcast_create_note(user_id.value(), json, ApVisibility::Public, vec![])
             .await?;
 
         Ok(())