refactor: prepare for free ticket

app/models/order.rb

@@ -116,6 +116,11 @@ class Order < ApplicationRecord
     promoter_payout_cents / 100.0
   end
 
+  # Check if order contains only free tickets
+  def free?
+    total_amount_cents == 0
+  end
+
   # Create Stripe invoice for accounting records
   #
   # This method creates a post-payment invoice in Stripe for accounting purposes