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/membership/member.ex

@@ -471,11 +471,12 @@ defmodule Mv.Membership.Member do
       end
     end
 
-    # Validate required custom fields
-    validate fn changeset, _ ->
+    # Validate required custom fields (actor from validation context only; no fallback)
+    validate fn changeset, context ->
       provided_values = provided_custom_field_values(changeset)
+      actor = context.actor
 
-      case Mv.Membership.list_required_custom_fields() do
+      case Mv.Membership.list_required_custom_fields(actor: actor) do
         {:ok, required_custom_fields} ->
           missing_fields = missing_required_fields(required_custom_fields, provided_values)
 

lib/membership/membership.ex

@@ -155,23 +155,31 @@ defmodule Mv.Membership do
   Lists only required custom fields.
 
   This is an optimized version that filters at the database level instead of
-  loading all custom fields and filtering in memory.
+  loading all custom fields and filtering in memory. Requires an actor for
+  authorization (CustomField read policy).
+
+  ## Options
+
+    - `:actor` - Required. The actor for authorization (e.g. current user).
+      All roles can read CustomField; actor must have a valid role.
 
   ## Returns
 
     - `{:ok, required_custom_fields}` - List of required custom fields
-    - `{:error, error}` - Error reading custom fields
+    - `{:error, error}` - Error reading custom fields (e.g. Forbidden when no actor)
 
   ## Examples
 
-      iex> {:ok, required_fields} = Mv.Membership.list_required_custom_fields()
+      iex> {:ok, required_fields} = Mv.Membership.list_required_custom_fields(actor: actor)
       iex> Enum.all?(required_fields, & &1.required)
       true
   """
-  def list_required_custom_fields do
+  def list_required_custom_fields(opts \\ []) do
+    actor = Keyword.get(opts, :actor)
+
     Mv.Membership.CustomField
     |> Ash.Query.filter(expr(required == true))
-    |> Ash.read(domain: __MODULE__)
+    |> Ash.read(domain: __MODULE__, actor: actor)
   end
 
   @doc """