fix(users): add include_all param to UserListView for contributor search

Superusers (admins) were excluded by the is_superuser=False filter,
making them unsearchable in the contributor picker. Pass include_all=1
to bypass this filter when searching for event contributors.

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

admin_api/views.py

@@ -585,7 +585,8 @@ class UserListView(APIView):
         from django.contrib.auth import get_user_model
         from django.db.models import Q
         User = get_user_model()
-        qs = User.objects.filter(is_superuser=False)
+        include_all = request.query_params.get('include_all', '0') == '1'
+        qs = User.objects.all() if include_all else User.objects.filter(is_superuser=False)
 
         # Server-side search
         search = request.query_params.get('search', '').strip()