Increase Express JSON body limit to 5MB

Default 100KB was silently rejecting hero image uploads (~456KB base64).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server.js

@@ -4,7 +4,7 @@ const crypto = require('crypto');
 const path = require('path');
 
 const app = express();
-app.use(express.json());
+app.use(express.json({ limit: '5mb' }));
 app.use(cookieParser());
 
 const PORT = process.env.PORT || 3000;