fix: HIGH+MEDIUM bugs from full audit - listen error, JWT, upload, mailer, webhook

backend/server.js

@@ -125,7 +125,7 @@ app.use((err, req, res, next) => {
   res.status(500).json({ error: '服务器内部错误' });
 });
 
-if (isInstalled()) console.log(`Server running at http://localhost:${PORT}`);
-else console.log(`System not installed. Visit http://localhost:${PORT}/#/install`);
-
-app.listen(PORT);
+app.listen(PORT, () => {
+  if (isInstalled()) console.log(`Server running at http://localhost:${PORT}`);
+  else console.log(`System not installed. Visit http://localhost:${PORT}/#/install`);
+}).on('error', (err) => { console.error('Failed to start:', err.message); process.exit(1); });