fix(backend): use correct Express 5 wildcard syntax /{*splat}

backend/src/index.ts

@@ -51,7 +51,7 @@ app.post('/_cron/tick', (req, res) => {
 const publicDir = path.join(__dirname, '../public');
 app.use(express.static(publicDir));
 
-app.get('*', (req, res, next) => {
+app.get('/{*splat}', (req, res, next) => {
   if (req.path.startsWith(apiRoot)) return next();
   if (req.path === '/_cron/tick') return next();