add paper trail

app/models/balance.rb

@@ -22,6 +22,8 @@
 #  fk_rails_...  (user_id => users.id)
 #
 class Balance < ApplicationRecord
+  include Trackable
+
   belongs_to :user
   belongs_to :currency
 

app/models/concerns/trackable.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+module Trackable
+  extend ActiveSupport::Concern
+
+  included do
+    has_paper_trail
+  end
+end

app/models/fiat_balance.rb

@@ -20,6 +20,8 @@
 #  fk_rails_...  (user_id => users.id)
 #
 class FiatBalance < ApplicationRecord
+  include Trackable
+
   belongs_to :user
 
   monetize :amount_cents