Member: skip required custom fields validation for set_vereinfacht_contact_id

Validation runs only for create_member and update_member so Vereinfacht
sync (which only sets vereinfacht_contact_id) no longer fails with
Required custom fields missing.

lib/membership/member.ex

@@ -500,14 +500,17 @@ defmodule Mv.Membership.Member do
       end
     end
 
-    # Validate required custom fields (actor from validation context only; no fallback)
+    # Validate required custom fields (actor from validation context only; no fallback).
+    # Only for create_member/update_member; skip for set_vereinfacht_contact_id (internal sync
+    # only sets vereinfacht_contact_id; custom fields were already validated and saved).
     validate fn changeset, context ->
                provided_values = provided_custom_field_values(changeset)
                actor = context.actor
 
                case Mv.Membership.list_required_custom_fields(actor: actor) do
                  {:ok, required_custom_fields} ->
-          missing_fields = missing_required_fields(required_custom_fields, provided_values)
+                   missing_fields =
+                     missing_required_fields(required_custom_fields, provided_values)
 
                    if Enum.empty?(missing_fields) do
                      :ok
@@ -543,7 +546,8 @@ defmodule Mv.Membership.Member do
                     message:
                       "Unable to validate required custom fields. Please try again or contact support."}
                end
-    end
+             end,
+             where: [action_is([:create_member, :update_member])]
 
     # Validate member fields that are marked as required in settings or by Vereinfacht.
     # When settings cannot be loaded, we still enforce email + Vereinfacht-required fields.