feat: add systemd sdnotify support (ready, watchdog, stopping)

- Add github.com/coreos/go-systemd/v22/daemon dependency
- Signal SdNotifyReady after configuration is loaded
- Start watchdog goroutine that pings systemd every WatchdogSec/2
- Signal SdNotifyStopping during graceful shutdown
- Update systemd unit file:
  - Type=notify (instead of simple)
  - WatchdogSec=30 (auto-restart if service hangs)
  - NotifyAccess=main (only main process can notify)

Benefits:
- systemd knows when service is truly ready
- Automatic detection of hung/frozen service
- Better integration with systemd supervision
- More accurate service status reporting

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

cmd/ja4sentinel/main.go

@@ -10,6 +10,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/coreos/go-systemd/v22/daemon"
 	"ja4sentinel/api"
 	"ja4sentinel/internal/capture"
 	"ja4sentinel/internal/config"
@@ -66,6 +67,42 @@ func main() {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
+	// Signal readiness to systemd
+	if _, err := daemon.SdNotify(false, daemon.SdNotifyReady); err != nil {
+		appLogger.Warn("main", "Failed to send READY notification to systemd", map[string]string{
+			"error": err.Error(),
+		})
+	}
+
+	// Start watchdog goroutine if enabled
+	watchdogInterval, err := daemon.SdWatchdogEnabled(false)
+	if err != nil {
+		appLogger.Warn("main", "Failed to check watchdog status", map[string]string{
+			"error": err.Error(),
+		})
+	}
+	if watchdogInterval > 0 {
+		appLogger.Info("main", "systemd watchdog enabled", map[string]string{
+			"interval": watchdogInterval.String(),
+		})
+		go func() {
+			ticker := time.NewTicker(watchdogInterval / 2)
+			defer ticker.Stop()
+			for {
+				select {
+				case <-ticker.C:
+					if _, err := daemon.SdNotify(false, daemon.SdNotifyWatchdog); err != nil {
+						appLogger.Warn("main", "Failed to send WATCHDOG notification", map[string]string{
+							"error": err.Error(),
+						})
+					}
+				case <-ctx.Done():
+					return
+				}
+			}
+		}()
+	}
+
 	// Setup signal handling
 	sigChan := make(chan os.Signal, 1)
 	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
@@ -176,6 +213,14 @@ func main() {
 
 	// Graceful shutdown
 	appLogger.Info("main", "Shutting down...", nil)
+
+	// Signal stopping to systemd
+	if _, err := daemon.SdNotify(false, daemon.SdNotifyStopping); err != nil {
+		appLogger.Warn("main", "Failed to send STOPPING notification to systemd", map[string]string{
+			"error": err.Error(),
+		})
+	}
+
 	cancel()
 
 	// Close components