Add Actor.permission_set_name/1 and admin?/1 for consistent capability checks

- Actor.permission_set_name(actor) returns role's permission set (supports nil role load).
- Actor.admin?(actor) returns true for system user or admin permission set.
- ActorIsAdmin policy check delegates to Actor.admin?/1.

lib/mv/authorization/checks/actor_is_admin.ex

@@ -3,20 +3,15 @@ defmodule Mv.Authorization.Checks.ActorIsAdmin do
   Policy check: true when the actor's role has permission_set_name "admin".
 
   Used to restrict actions (e.g. User.update_user for member link/unlink) to admins only.
+  Delegates to `Mv.Authorization.Actor.admin?/1` for consistency.
   """
   use Ash.Policy.SimpleCheck
 
+  alias Mv.Authorization.Actor
+
   @impl true
   def describe(_opts), do: "actor has admin permission set"
 
   @impl true
-  def match?(nil, _context, _opts), do: false
-
-  def match?(actor, _context, _opts) do
-    ps_name =
-      get_in(actor, [Access.key(:role), Access.key(:permission_set_name)]) ||
-        get_in(actor, [Access.key("role"), Access.key("permission_set_name")])
-
-    ps_name == "admin"
-  end
+  def match?(actor, _context, _opts), do: Actor.admin?(actor)
 end