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/routes/users.js

@@ -27,6 +27,7 @@ router.post('/', authenticate, requireRole('owner','admin'), async (req, res) =>
   if (role === 'owner' && req.user.role !== 'owner') return res.status(403).json({ error: '仅服主可创建服主账号' });
   if (password.length < 6) return res.status(400).json({ error: '密码至少6位' });
   if (await getRow('SELECT id FROM users WHERE username = ?', [username])) return res.status(400).json({ error: '用户名已存在' });
+  if (await getRow('SELECT id FROM users WHERE email = ?', [email])) return res.status(400).json({ error: '邮箱已被使用' });
   const hashed = bcrypt.hashSync(password, 10);
   const r = await query('INSERT INTO users(username, password, email, game_name, game_uid, role, source, active, email_verified) VALUES (?,?,?,?,?,?,?,1,1)', [username, hashed, email, game_name, game_uid, role, source || 'netease']);
   await query("INSERT INTO audit_logs(user_id, username, action, entity_type, entity_id, details) VALUES (?,?,?,?,?,?)", [req.user.id, req.user.username, 'create_user', 'user', r.insertId, `创建用户 ${username}`]);