Feat: Enforce rate limit (1 post per 30s)

server.js

@@ -34,6 +34,10 @@ async function savePost(post) {
 const { protect, challenge } = createExpressMiddleware({
   secret: 'ai-twitter-secret-key-123',
   difficulty: 'medium',
+  rateLimit: {
+    maxAttempts: 1,
+    windowMs: 30000 // 30 seconds
+  }
 });
 
 // --- API Endpoints ---