Fix Vercel function request body parsing

🐛 Bug Fix:
- Replace req.json() with req.text() + JSON.parse() for Vercel compatibility
- Add proper error handling for malformed JSON requests
- Fix authentication endpoints (login/register) body parsing
- Fix admin users endpoint body parsing

🔧 Technical Details:
- Vercel functions don't support req.json() method directly
- Use req.text() to get raw body content then parse manually
- Add try/catch blocks for JSON parsing errors
- Maintain same API interface and error responses

✅ Endpoints Fixed:
- /api/auth/register - User registration
- /api/auth/login - User authentication
- /api/admin/users - Admin user management

This resolves the 'req.json is not a function' error in production.

api/admin/users.ts

@@ -105,7 +105,20 @@ export default async function handler(req: NextRequest) {
         });
       }
 
-      const body = await req.json();
+      // Parse request body
+      let body: { isActive?: boolean; roles?: string[] };
+      try {
+        const bodyText = await req.text();
+        body = JSON.parse(bodyText);
+      } catch (parseError) {
+        return new NextResponse(JSON.stringify({ 
+          error: 'Invalid JSON in request body' 
+        }), {
+          status: 400,
+          headers: { 'Content-Type': 'application/json' },
+        });
+      }
+
       const { isActive, roles } = body;
 
       // Update user status

api/auth/login.ts

@@ -24,7 +24,20 @@ export default async function handler(req: NextRequest) {
   const client = await pool.connect();
   
   try {
-    const body: LoginRequest = await req.json();
+    // Parse request body
+    let body: LoginRequest;
+    try {
+      const bodyText = await req.text();
+      body = JSON.parse(bodyText);
+    } catch (parseError) {
+      return new NextResponse(JSON.stringify({ 
+        error: 'Invalid JSON in request body' 
+      }), {
+        status: 400,
+        headers: { 'Content-Type': 'application/json' },
+      });
+    }
+
     const { username, password } = body;
 
     // Validate input

api/auth/register.ts

@@ -27,7 +27,20 @@ export default async function handler(req: NextRequest) {
   const client = await pool.connect();
   
   try {
-    const body: RegisterRequest = await req.json();
+    // Parse request body
+    let body: RegisterRequest;
+    try {
+      const bodyText = await req.text();
+      body = JSON.parse(bodyText);
+    } catch (parseError) {
+      return new NextResponse(JSON.stringify({ 
+        error: 'Invalid JSON in request body' 
+      }), {
+        status: 400,
+        headers: { 'Content-Type': 'application/json' },
+      });
+    }
+
     const { username, email, password, firstName, lastName } = body;
 
     // Validate input