refactor: address review comments for join request settings

lib/membership/setting.ex

@@ -60,6 +60,10 @@ defmodule Mv.Membership.Setting do
     domain: Mv.Membership,
     data_layer: AshPostgres.DataLayer
 
+  # Used in join_form_field_ids validation (compile-time to avoid recompiling regex and list on every validation)
+  @uuid_pattern ~r/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i
+  @valid_join_form_member_fields Mv.Constants.member_fields() |> Enum.map(&Atom.to_string/1)
+
   postgres do
     table "settings"
     repo Mv.Repo
@@ -255,16 +259,10 @@ defmodule Mv.Membership.Setting do
                field_ids = Ash.Changeset.get_attribute(changeset, :join_form_field_ids)
 
                if is_list(field_ids) and field_ids != [] do
-                 valid_member_fields =
-                   Mv.Constants.member_fields() |> Enum.map(&Atom.to_string/1)
-
-                 uuid_pattern =
-                   ~r/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i
-
                  invalid_ids =
                    Enum.reject(field_ids, fn id ->
                      is_binary(id) and
-                       (id in valid_member_fields or Regex.match?(uuid_pattern, id))
+                       (id in @valid_join_form_member_fields or Regex.match?(@uuid_pattern, id))
                    end)
 
                  if Enum.empty?(invalid_ids) do
@@ -442,6 +440,7 @@ defmodule Mv.Membership.Setting do
 
     attribute :join_form_field_ids, {:array, :string} do
       allow_nil? true
+      default []
       public? true
 
       description "Ordered list of field IDs shown on the join form. Each entry is a member field name (e.g. 'email') or a custom field UUID. Email is always present after normalization."