fix: harden DynDNS recovery and publish v1.5.1

internal/updateprotocol/protocol.go

@@ -30,6 +30,8 @@ type Request struct {
 }
 
 type Status struct {
+	// Optional capability; older v2 agents require registry credentials.
+	AnonymousPull bool      `json:"anonymous_pull,omitempty"`
 	Phase         string    `json:"phase"`
 	TargetVersion string    `json:"target_version,omitempty"`
 	Message       string    `json:"message,omitempty"`