fix: CustomField policies, no system-actor fallback, guidelines

- Tests and UI pass actor for CustomField create/read/destroy; seeds use actor
- Member required-custom-fields validation uses context.actor only (no fallback)
- CODE_GUIDELINES: add rule forbidding system-actor fallbacks

lib/mv_web/live/custom_field_live/index_component.ex

@@ -176,6 +176,13 @@ defmodule MvWeb.CustomFieldLive.IndexComponent do
     """
   end
 
+  defp stream_custom_fields(actor) do
+    case Ash.read(Mv.Membership.CustomField, actor: actor) do
+      {:ok, custom_fields} -> custom_fields
+      {:error, _} -> []
+    end
+  end
+
   @impl true
   def update(assigns, socket) do
     # Track previous show_form state to detect when form is closed
@@ -207,7 +214,7 @@ defmodule MvWeb.CustomFieldLive.IndexComponent do
      |> assign_new(:show_delete_modal, fn -> false end)
      |> assign_new(:custom_field_to_delete, fn -> nil end)
      |> assign_new(:slug_confirmation, fn -> "" end)
-     |> stream(:custom_fields, Ash.read!(Mv.Membership.CustomField), reset: true)}
+     |> stream(:custom_fields, stream_custom_fields(assigns[:actor]), reset: true)}
   end
 
   @impl true
@@ -226,7 +233,8 @@ defmodule MvWeb.CustomFieldLive.IndexComponent do
 
   @impl true
   def handle_event("edit_custom_field", %{"id" => id}, socket) do
-    custom_field = Ash.get!(Mv.Membership.CustomField, id)
+    actor = socket.assigns[:actor]
+    custom_field = Ash.get!(Mv.Membership.CustomField, id, actor: actor)
 
     # Only send event if form was not already open
     if not socket.assigns[:show_form] do
@@ -242,7 +250,13 @@ defmodule MvWeb.CustomFieldLive.IndexComponent do
 
   @impl true
   def handle_event("prepare_delete", %{"id" => id}, socket) do
-    custom_field = Ash.get!(Mv.Membership.CustomField, id, load: [:assigned_members_count])
+    actor = socket.assigns[:actor]
+
+    custom_field =
+      Ash.get!(Mv.Membership.CustomField, id,
+        load: [:assigned_members_count],
+        actor: actor
+      )
 
     {:noreply,
      socket
@@ -259,9 +273,10 @@ defmodule MvWeb.CustomFieldLive.IndexComponent do
   @impl true
   def handle_event("confirm_delete", _params, socket) do
     custom_field = socket.assigns.custom_field_to_delete
+    actor = socket.assigns[:actor]
 
     if socket.assigns.slug_confirmation == custom_field.slug do
-      case Ash.destroy(custom_field) do
+      case Ash.destroy(custom_field, actor: actor) do
         :ok ->
           send(self(), {:custom_field_deleted, custom_field})