feat: Add promotion code functionality to ticket orders

app/controllers/orders_controller.rb

@@ -126,6 +126,20 @@ class OrdersController < ApplicationController
     @total_amount = @order.total_amount_cents
     @expiring_soon = @order.expiring_soon?
 
+    # Handle promotion code application
+    if params[:promotion_code].present?
+      promotion_code = PromotionCode.valid.find_by(code: params[:promotion_code].upcase)
+      if promotion_code
+        # Apply the promotion code to the order
+        @order.promotion_codes << promotion_code
+        @order.calculate_total!
+        @total_amount = @order.total_amount_cents
+        flash.now[:notice] = "Code promotionnel appliqué: #{promotion_code.code}"
+      else
+        flash.now[:alert] = "Code promotionnel invalide"
+      end
+    end
+
     # For free orders, automatically mark as paid and redirect to success
     if @order.free?
       @order.mark_as_paid!