fix: 使用 setsid 确保重启命令独立于父进程执行

问题原因：
- cmd.Start() 启动的子进程与父进程在同一会话中
- 当 systemctl restart 发送 SIGTERM 给父进程时
- 子进程可能也会被终止，导致重启命令无法完成

修复内容：
- 使用 setsid 创建新会话，子进程完全独立于父进程
- 分离标准输入/输出/错误流
- 确保即使父进程被 kill，重启命令仍能执行完成

backend/internal/pkg/sysutil/restart.go

@@ -67,7 +67,17 @@ func RestartService() error {
 	// The sub2api user has NOPASSWD sudo access for systemctl commands
 	// (configured by install.sh in /etc/sudoers.d/sub2api).
 	// Use -n (non-interactive) to prevent sudo from waiting for password input
-	cmd := exec.Command(sudoPath, "-n", systemctlPath, "restart", serviceName)
+	//
+	// Use setsid to create a new session, ensuring the child process
+	// survives even if the parent process is killed by systemctl restart
+	setsidPath := findExecutable("setsid")
+	cmd := exec.Command(setsidPath, sudoPath, "-n", systemctlPath, "restart", serviceName)
+
+	// Detach from parent's stdio to ensure clean separation
+	cmd.Stdin = nil
+	cmd.Stdout = nil
+	cmd.Stderr = nil
+
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("failed to initiate service restart: %w", err)
 	}