CustomField policies: actor required, no system-actor fallback, error handling

- list_required_custom_fields: require actor (two clauses, no default)
- Member validation: use context.actor only, differentiate Forbidden vs transient errors
- stream_custom_fields: log + send flash on error instead of returning []
- GlobalSettingsLive: handle_info for custom_fields_load_error, put_flash
- Seeds: use Membership.update_member with actor, format

lib/membership/membership.ex

@@ -14,7 +14,7 @@ defmodule Mv.Membership do
   The domain exposes these main actions:
   - Member CRUD: `create_member/1`, `list_members/0`, `update_member/2`, `destroy_member/1`
   - Custom field value management: `create_custom_field_value/1`, `list_custom_field_values/0`, etc.
-  - Custom field management: `create_custom_field/1`, `list_custom_fields/0`, `list_required_custom_fields/0`, etc.
+  - Custom field management: `create_custom_field/1`, `list_custom_fields/0`, `list_required_custom_fields/1`, etc.
   - Settings management: `get_settings/0`, `update_settings/2`, `update_member_field_visibility/2`, `update_single_member_field_visibility/3`
   - Group management: `create_group/1`, `list_groups/0`, `update_group/2`, `destroy_group/1`
   - Member-group associations: `create_member_group/1`, `list_member_groups/0`, `destroy_member_group/1`
@@ -156,7 +156,7 @@ defmodule Mv.Membership do
 
   This is an optimized version that filters at the database level instead of
   loading all custom fields and filtering in memory. Requires an actor for
-  authorization (CustomField read policy).
+  authorization (CustomField read policy). Callers must pass `actor:`; no default.
 
   ## Options
 
@@ -166,22 +166,26 @@ defmodule Mv.Membership do
   ## Returns
 
     - `{:ok, required_custom_fields}` - List of required custom fields
-    - `{:error, error}` - Error reading custom fields (e.g. Forbidden when no actor)
+    - `{:error, :missing_actor}` - When actor is nil (caller must pass actor)
+    - `{:error, error}` - Error reading custom fields (e.g. Forbidden)
 
   ## Examples
 
       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(opts \\ []) do
-    actor = Keyword.get(opts, :actor)
 
+      iex> Mv.Membership.list_required_custom_fields(actor: nil)
+      {:error, :missing_actor}
+  """
+  def list_required_custom_fields(actor: actor) when not is_nil(actor) do
     Mv.Membership.CustomField
     |> Ash.Query.filter(expr(required == true))
     |> Ash.read(domain: __MODULE__, actor: actor)
   end
 
+  def list_required_custom_fields(actor: nil), do: {:error, :missing_actor}
+
   @doc """
   Updates the member field visibility configuration.