Add debug logging to promote-user endpoint

🐛 Debug:
- Log request body type and content
- Identify why JSON parsing is failing
- Temporary debug version to diagnose issue

api/admin/promote-user.js

@@ -11,6 +11,9 @@ export default async function handler(req, res) {
   }
 
   try {
+    console.log('Request body type:', typeof req.body);
+    console.log('Request body:', req.body);
+    
     const { username, userId } = req.body;
 
     if (!username && !userId) {