docs: add Clerk setup guide and improve auth error messages

- Create CLERK_SETUP.md with step-by-step instructions
- Improve error message to indicate Clerk configuration issue
- Remove debug logging from auth middleware
- Update error handling to be more helpful

The 401 error is caused by missing Clerk Secret Key in server/.env

server/src/middleware/auth.ts

@@ -48,7 +48,7 @@ export const requireAuth = async (
     return res.status(401).json({
       error: {
         code: 'UNAUTHORIZED',
-        message: 'Authentication failed',
+        message: 'Authentication failed. Please check that CLERK_SECRET_KEY is configured correctly in server/.env',
         timestamp: new Date().toISOString(),
       },
     });