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/infrastructure/ap_inbox_handlers.py

@@ -253,6 +253,7 @@ async def handle_follow(
             actor_profile_id=actor_row.id,
             notification_type="follow",
             from_remote_actor_id=ra.id,
+            app_domain=app_domain,
         )
         session.add(notif)