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/nats/src/subject.rs

@@ -13,6 +13,7 @@ pub fn event_to_subject(prefix: &str, event: &DomainEvent) -> String {
         DomainEvent::WatchlistEntryAdded { .. } => "watchlist.entry.added",
         DomainEvent::WatchlistEntryRemoved { .. } => "watchlist.entry.removed",
         DomainEvent::FollowAccepted { .. } => "follow.accepted",
+        DomainEvent::BackfillFollower { .. } => "backfill.follower",
     };
     format!("{prefix}.{suffix}")
 }