feat(promotion code): Promotion code system done

I added the features for users to use promotion code
and for promoters to create on their events.
May be rewrite to discount code?

app/models/order.rb

@@ -90,10 +90,34 @@ class Order < ApplicationRecord
     end
   end
 
-  # Calculate total from ticket prices only (platform fee deducted from promoter payout)
+  # Calculate total from ticket prices minus promotion code discounts
   def calculate_total!
     ticket_total = tickets.sum(:price_cents)
-    update!(total_amount_cents: ticket_total)
+    discount_total = promotion_codes.sum(:discount_amount_cents)
+
+    # Ensure total doesn't go below zero
+    final_total = [ticket_total - discount_total, 0].max
+    update!(total_amount_cents: final_total)
+  end
+
+  # Subtotal amount before discounts
+  def subtotal_amount_cents
+    tickets.sum(:price_cents)
+  end
+
+  # Subtotal amount in euros
+  def subtotal_amount_euros
+    subtotal_amount_cents / 100.0
+  end
+
+  # Total discount amount from all promotion codes
+  def discount_amount_cents
+    promotion_codes.sum(:discount_amount_cents)
+  end
+
+  # Discount amount in euros
+  def discount_amount_euros
+    discount_amount_cents / 100.0
   end
 
   # Platform fee: €0.50 fixed + 1.5% of ticket price, per ticket