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>

go.sum

@@ -1,3 +1,5 @@
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@@ -13,10 +15,13 @@ golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJ
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