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/services/stripe_invoice_service.rb

@@ -166,6 +166,23 @@ class StripeInvoiceService
       })
     end
 
+    # Add promotion code discounts as negative line items
+    @order.promotion_codes.each do |promo_code|
+      Stripe::InvoiceItem.create({
+        customer: customer.id,
+        invoice: invoice.id,
+        amount: -promo_code.discount_amount_cents, # Negative amount for discount
+        currency: "eur",
+        description: "Réduction promotionnelle (Code: #{promo_code.code})",
+        metadata: {
+          promotion_code_id: promo_code.id,
+          promotion_code: promo_code.code,
+          discount_amount_cents: promo_code.discount_amount_cents,
+          type: "promotion_discount"
+        }
+      })
+    end
+
     # No service fee on customer invoice; platform fee deducted from promoter payout
   end