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>

packaging/systemd/ja4sentinel.service

@@ -5,13 +5,15 @@ After=network.target
 Wants=network-online.target
 
 [Service]
-Type=simple
+Type=notify
 User=root
 Group=root
 WorkingDirectory=/var/lib/ja4sentinel
 ExecStart=/usr/bin/ja4sentinel --config /etc/ja4sentinel/config.yml
 Restart=on-failure
 RestartSec=5
+WatchdogSec=30
+NotifyAccess=main
 Environment=JA4SENTINEL_LOG_LEVEL=info
 
 # Security hardening (compatible with root for packet capture)