Show per-app actor in follow notifications on Hub

Add app_domain to APNotification model and NotificationDTO so follow
notifications display "followed you on blog" instead of just "followed
you" when the follow targets a per-app actor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/services/federation_impl.py

@@ -1620,6 +1620,7 @@ class SqlFederationService:
                 target_content_preview=preview,
                 created_at=notif.created_at,
                 read=notif.read,
+                app_domain=notif.app_domain,
             ))
         return items