fix: stabilize software updates

internal/updateprotocol/protocol.go

@@ -24,7 +24,9 @@ type Request struct {
 	Version          string `json:"version"`
 	CurrentVersion   string `json:"current_version"`
 	RegistryUsername string `json:"registry_username"`
-	RegistryToken    string `json:"registry_token"`
+	// RegistryToken keeps the v2 wire name for compatibility and carries either
+	// a registry password or an API token.
+	RegistryToken string `json:"registry_token"`
 }
 
 type Status struct {