fix: async route rejection crashes process - global async error wrap + email dup check

- users.js: check email uniqueness before insert (was ER_DUP_ENTRY uncaught crash)
- server.js: wrapAsyncRouter - Express 4 doesn't catch async handler rejections,
  route throws now go to error middleware (500 JSON) instead of uncaughtException

backend/server.js

@@ -131,11 +131,53 @@ function loadBusinessRoutes() {
   businessRouter.use('/external', require('./routes/external'));
 
   businessRouter.get('/verify', (req, res) => res.redirect(`/#/verify?token=${req.query.token}`));
+
+  wrapAsyncRouter(businessRouter);
 }
 
 if (isInstalled()) loadBusinessRoutes();
 global.__loadBusinessRoutes = loadBusinessRoutes;
 
+// Express 4 不自动捕获 async handler 的 rejection, 一旦 Promise 拒绝会 uncaughtException 崩掉整个进程。
+// 对所有已挂载路由做递归包装: rejection → next(err) → 统一错误中间件(500 JSON)。
+function wrapAsyncRouter(router) {
+  if (!router || router.__asyncWrapped) return;
+  router.__asyncWrapped = true;
+  for (const layer of router.stack) {
+    if (layer.route) {
+      for (const rl of layer.route.stack) {
+        const h = rl.handle;
+        if (typeof h === 'function' && h.length <= 3) {
+          rl.handle = (req, res, next) => {
+            try { const p = h(req, res, next); if (p && p.catch) p.catch(next); } catch (e) { next(e); }
+          };
+        }
+      }
+    } else {
+      const h = layer.handle;
+      if (typeof h === 'function' && h.stack) { wrapAsyncRouter(h); continue; }
+      if (typeof h === 'function' && h.length <= 3) {
+        layer.handle = (req, res, next) => {
+          try { const p = h(req, res, next); if (p && p.catch) p.catch(next); } catch (e) { next(e); }
+        };
+      }
+    }
+  }
+}
+wrapAsyncRouter(businessRouter);
+wrapAsyncRouter(installRoutes);
+// 顶层 async 路由同样包装
+for (const layer of app._router.stack) {
+  if (layer.route && layer.route.path === '/' && layer.route.stack[0]) {
+    const h = layer.route.stack[0].handle;
+    if (typeof h === 'function' && h.length <= 3) {
+      layer.route.stack[0].handle = (req, res, next) => {
+        try { const p = h(req, res, next); if (p && p.catch) p.catch(next); } catch (e) { next(e); }
+      };
+    }
+  }
+}
+
 app.get('/api/health', (req, res) => res.json({ status: 'ok', installed: isInstalled() }));
 
 app.get('/', async (req, res) => {