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/domain/src/events.rs

@@ -61,6 +61,10 @@ pub enum DomainEvent {
         remote_actor_url: String,
         outbox_url: String,
     },
+    BackfillFollower {
+        owner_user_id: UserId,
+        follower_inbox_url: String,
+    },
 }
 
 #[async_trait]