Only admins or the linked user may change a linked member's email.
- New validation EmailChangePermission (uses Actor.admin?, Loader.get_linked_user).
- Register on Member update_member; docs and gettext.
- 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.