fix: 修复 sudo 在非交互模式下无法执行的问题

问题原因：
- sudo 命令没有 -n 选项
- 在后台服务中，sudo 会尝试从终端读取密码
- 由于没有终端，命令静默失败

修复内容：
- 添加 sudo -n 选项强制非交互模式
- 如果需要密码会立即失败并返回错误，而不是挂起

backend/internal/pkg/sysutil/restart.go

@@ -32,7 +32,8 @@ func RestartService() error {
 
 	// The sub2api user has NOPASSWD sudo access for systemctl commands
 	// (configured by install.sh in /etc/sudoers.d/sub2api).
-	cmd := exec.Command("sudo", "systemctl", "restart", serviceName)
+	// Use -n (non-interactive) to prevent sudo from waiting for password input
+	cmd := exec.Command("sudo", "-n", "systemctl", "restart", serviceName)
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("failed to initiate service restart: %w", err)
 	}