fix: add explicit role field to admin login response

Makes auth role logic consistent between admin (role: 'admin')
and sub-account (role: 'iam_user') logins.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/apps/accounts/serializers.py

@@ -13,6 +13,10 @@ class UserInfoSerializer(serializers.Serializer):
     is_active = serializers.BooleanField()
     date_joined = serializers.DateTimeField()
     last_login = serializers.DateTimeField()
+    role = serializers.SerializerMethodField()
+
+    def get_role(self, obj):
+        return 'admin'
 
 
 class ChangePasswordSerializer(serializers.Serializer):