Add System Actor helper for systemic operations

Introduce Mv.Helpers.SystemActor module with lazy loading
for operations that must always run regardless of user permissions.
System actor has admin role and auto-creates in test environment.

lib/mv/application.ex

@@ -14,6 +14,7 @@ defmodule Mv.Application do
       {DNSCluster, query: Application.get_env(:mv, :dns_cluster_query) || :ignore},
       {Phoenix.PubSub, name: Mv.PubSub},
       {AshAuthentication.Supervisor, otp_app: :my},
+      Mv.Helpers.SystemActor,
       # Start a worker by calling: Mv.Worker.start_link(arg)
       # {Mv.Worker, arg},
       # Start to serve requests, typically the last entry