Fix promote-user endpoint request body parsing

🔧 Fix:
- Use req.body directly instead of req.text() + JSON.parse()
- Match pattern used in working auth endpoints
- Should resolve NOT_FOUND and parsing issues

api/admin/promote-user.js

@@ -11,8 +11,7 @@ export default async function handler(req, res) {
   }
 
   try {
-    const body = JSON.parse(req.body || '{}');
-    const { username, userId } = body;
+    const { username, userId } = req.body;
 
     if (!username && !userId) {
       return res.status(400).json({