Add CustomFieldValueCreateScope check for create actions

Ash cannot apply filters to create; this check enforces :linked/:all scope
via strict_check only (no filter).

lib/mv/authorization/checks/custom_field_value_create_scope.ex

@@ -0,0 +1,64 @@
+defmodule Mv.Authorization.Checks.CustomFieldValueCreateScope do
+  @moduledoc """
+  Policy check for CustomFieldValue create actions only.
+
+  Use this for create instead of HasPermission because Ash cannot apply
+  filters to create actions ("Cannot use a filter to authorize a create").
+  This check performs the same scope logic as HasPermission for create
+  (PermissionSets + :linked/:all) but only implements strict_check, so it
+  never adds a filter.
+
+  Used in CustomFieldValue policies:
+    policy action_type(:create) do
+      authorize_if Mv.Authorization.Checks.CustomFieldValueCreateScope
+    end
+  """
+  use Ash.Policy.Check
+  alias Mv.Authorization.PermissionSets
+  require Logger
+
+  @impl true
+  def describe(_opts),
+    do: "CustomFieldValue create allowed by permission set scope (:linked or :all)"
+
+  @impl true
+  def strict_check(actor, authorizer, _opts) do
+    actor = ensure_role_loaded(actor)
+
+    with %{role: %{permission_set_name: ps_name}} when not is_nil(ps_name) <- actor,
+         {:ok, ps_atom} <- PermissionSets.permission_set_name_to_atom(ps_name),
+         permissions <- PermissionSets.get_permissions(ps_atom),
+         perm <- find_custom_field_value_create(permissions.resources) do
+      case perm do
+        nil -> {:ok, false}
+        %{scope: :all} -> {:ok, true}
+        %{scope: :linked} -> {:ok, member_id_matches?(authorizer, actor)}
+      end
+    else
+      _ -> {:ok, false}
+    end
+  end
+
+  defp find_custom_field_value_create(resources) do
+    Enum.find(resources, fn p ->
+      p.resource == "CustomFieldValue" and p.action == :create and p.granted
+    end)
+  end
+
+  defp member_id_matches?(authorizer, actor) do
+    member_id = get_create_member_id(authorizer)
+    !is_nil(member_id) and member_id == actor.member_id
+  end
+
+  defp get_create_member_id(authorizer) do
+    changeset = authorizer.changeset || authorizer.subject
+
+    if changeset && function_exported?(Ash.Changeset, :get_attribute, 2) do
+      Ash.Changeset.get_attribute(changeset, :member_id)
+    else
+      nil
+    end
+  end
+
+  defp ensure_role_loaded(actor), do: Mv.Authorization.Actor.ensure_loaded(actor)
+end