feat: Add EnsureCentralService + improved tests

Linter-contributed improvements:
- EnsureCentralService() method: registers Central's own domain as a
  service from config, with cleanup on domain change
- Tests for EnsureCentralService: registration, idempotency, domain
  change cleanup, no-domain-is-noop
- Improved reconciliation tests with Central as a registered service
- Certbot handler cleanup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/api/v1/handlers.go

@@ -320,8 +320,9 @@ func (api *API) UpdateGlobalConfig(w http.ResponseWriter, r *http.Request) {
 
 	slog.Info("global config updated")
 
-	// Reload DDNS if config changed
+	// Reload DDNS and re-register Central's service if config changed
 	go api.reloadDDNSIfEnabled()
+	go api.EnsureCentralService()
 
 	respondMessage(w, http.StatusOK, "Config updated successfully")
 }