fix(promotion code): Cap the minimum invoice for Stripe

Stripe does not support negative invoices, so to
allow correct invoice generation, we apply dismiss
negative invoices.

app/models/order.rb

@@ -21,6 +21,9 @@ class Order < ApplicationRecord
   validates :payment_attempts, presence: true,
     numericality: { greater_than_or_equal_to: 0 }
 
+  # Custom validation to prevent duplicate promotion codes
+  validate :no_duplicate_promotion_codes
+
   # Stripe invoice ID for accounting records
   attr_accessor :stripe_invoice_id
 
@@ -188,4 +191,12 @@ class Order < ApplicationRecord
   def draft?
     status == "draft"
   end
+
+  # Prevent duplicate promotion codes on the same order
+  def no_duplicate_promotion_codes
+    promotion_code_ids = promotion_codes.map(&:id)
+    if promotion_code_ids.size != promotion_code_ids.uniq.size
+      errors.add(:promotion_codes, "ne peuvent pas contenir de codes en double")
+    end
+  end
 end