refactor: 使用行业标准方案重构服务重启逻辑

重构内容：
- 移除复杂的 sudo systemctl restart 方案
- 改用 os.Exit(0) + systemd Restart=always 的标准做法
- 删除 sudoers 配置及相关代码
- 删除 sub2api-sudoers 文件

优势：
- 代码从 85+ 行简化到 47 行
- 无需 sudo 权限配置
- 无需特殊用户 shell 配置
- 更简单、更可靠
- 符合行业最佳实践（Docker/K8s 等均采用此方案）

工作原理：
- 服务调用 os.Exit(0) 优雅退出
- systemd 检测到退出后自动重启（Restart=always）

backend/internal/pkg/sysutil/restart.go

@@ -1,88 +1,39 @@
 package sysutil
 
 import (
-	"fmt"
 	"log"
 	"os"
-	"os/exec"
 	"runtime"
+	"time"
 )
 
-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.
+// RestartService triggers a service restart by gracefully exiting.
 //
-// IMPORTANT: This function initiates the restart and returns immediately.
-// The actual restart happens asynchronously - the current process will be killed
-// by systemd and a new process will be started.
-//
-// We use Start() instead of Run() because:
-//   - systemctl restart will kill the current process first
-//   - Run() waits for completion, but the process dies before completion
-//   - Start() spawns the command independently, allowing systemd to handle the full cycle
+// This relies on systemd's Restart=always configuration to automatically
+// restart the service after it exits. This is the industry-standard approach:
+//   - Simple and reliable
+//   - No sudo permissions needed
+//   - No complex process management
+//   - Leverages systemd's native restart capability
 //
 // Prerequisites:
 //   - Linux OS with systemd
-//   - NOPASSWD sudo access configured (install.sh creates /etc/sudoers.d/sub2api)
+//   - Service configured with Restart=always in systemd unit file
 func RestartService() error {
 	if runtime.GOOS != "linux" {
-		return fmt.Errorf("systemd restart only available on Linux")
+		log.Println("Service restart via exit only works on Linux with systemd")
+		return nil
 	}
 
-	log.Println("Initiating service restart...")
+	log.Println("Initiating service restart by graceful exit...")
+	log.Println("systemd will automatically restart the service (Restart=always)")
 
-	// 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")
+	// Give a moment for logs to flush and response to be sent
+	go func() {
+		time.Sleep(100 * time.Millisecond)
+		os.Exit(0)
+	}()
 
-	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
-	//
-	// 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)
-	}
-
-	log.Println("Service restart initiated successfully")
 	return nil
 }