feat(promotion-code): Complete promotion code integration and testing

- Add comprehensive promotion code methods to Order model
- Implement Stripe invoice integration for promotion code discounts
- Display promotion codes on invoice with proper discount breakdown
- Fix and enhance all unit tests for promotion code functionality
- Add discount calculation with capping to prevent negative totals
- Ensure promotion codes work across entire order lifecycle

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/models/order.rb

@@ -96,7 +96,7 @@ class Order < ApplicationRecord
     discount_total = promotion_codes.sum(:discount_amount_cents)
 
     # Ensure total doesn't go below zero
-    final_total = [ticket_total - discount_total, 0].max
+    final_total = [ ticket_total - discount_total, 0 ].max
     update!(total_amount_cents: final_total)
   end
 
@@ -110,9 +110,9 @@ class Order < ApplicationRecord
     subtotal_amount_cents / 100.0
   end
 
-  # Total discount amount from all promotion codes
+  # Total discount amount from all promotion codes (capped at subtotal)
   def discount_amount_cents
-    promotion_codes.sum(:discount_amount_cents)
+    [ promotion_codes.sum(:discount_amount_cents), subtotal_amount_cents ].min
   end
 
   # Discount amount in euros