fix: harden backend stability and test production utilities

backend/middleware/error-handler.js

@@ -0,0 +1,20 @@
+function expressErrorHandler(err, req, res, next) {
+  if (res.headersSent) return next(err);
+
+  const requestedStatus = Number(err?.statusCode || err?.status);
+  const status = Number.isInteger(requestedStatus) && requestedStatus >= 400 && requestedStatus <= 599
+    ? requestedStatus
+    : 500;
+
+  console.error(`[未处理错误] ${req.method} ${req.originalUrl}`, {
+    message: err?.message,
+    stack: err?.stack
+  });
+
+  return res.status(status).json({
+    success: false,
+    message: status >= 500 ? '服务器内部错误' : (err?.message || '请求处理失败')
+  });
+}
+
+module.exports = { expressErrorHandler };