refactor: DRY - use Mv.Constants.custom_field_prefix() instead of string literals

lib/mv/constants.ex

@@ -18,5 +18,17 @@ defmodule Mv.Constants do
     :postal_code
   ]
 
+  @custom_field_prefix "custom_field_"
+
   def member_fields, do: @member_fields
+
+  @doc """
+  Returns the prefix used for custom field keys in field visibility maps.
+
+  ## Examples
+
+      iex> Mv.Constants.custom_field_prefix()
+      "custom_field_"
+  """
+  def custom_field_prefix, do: @custom_field_prefix
 end