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

@@ -3,12 +3,39 @@ package sysutil
 import (
 	"fmt"
 	"log"
+	"os"
 	"os/exec"
 	"runtime"
 )
 
 const serviceName = "sub2api"
 
+// findExecutable finds the full path of an executable
+// by checking common system paths
+func findExecutable(name string) string {
+	// First try exec.LookPath (uses current PATH)
+	if path, err := exec.LookPath(name); err == nil {
+		return path
+	}
+
+	// Fallback: check common paths
+	commonPaths := []string{
+		"/usr/bin/" + name,
+		"/bin/" + name,
+		"/usr/sbin/" + name,
+		"/sbin/" + name,
+	}
+
+	for _, path := range commonPaths {
+		if _, err := os.Stat(path); err == nil {
+			return path
+		}
+	}
+
+	// Return the name as-is and let exec fail with a clear error
+	return name
+}
+
 // RestartService triggers a service restart via systemd.
 //
 // IMPORTANT: This function initiates the restart and returns immediately.
@@ -30,10 +57,27 @@ func RestartService() error {
 
 	log.Println("Initiating service restart...")
 
+	// Find full paths for sudo and systemctl
+	// This ensures the commands work even if PATH is limited in systemd service
+	sudoPath := findExecutable("sudo")
+	systemctlPath := findExecutable("systemctl")
+
+	log.Printf("Using sudo: %s, systemctl: %s", sudoPath, systemctlPath)
+
 	// 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("sudo", "-n", "systemctl", "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)
 	}