fix(admin): include eventifyId in _serialize_user for users table

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

admin_api/views.py

@@ -526,6 +526,7 @@ def _serialize_user(u):
         avatar = ''
     return {
         'id': str(u.id),
+        'eventifyId': u.eventify_id or '',
         'name': full_name,
         'email': u.email,
         'phone': getattr(u, 'phone_number', '') or '',