feat: Add promotion code functionality to ticket orders

app/models/promotion_code.rb

@@ -0,0 +1,23 @@
+class PromotionCode < ApplicationRecord
+  # Validations
+  validates :code, presence: true, uniqueness: true
+  validates :discount_amount_cents, numericality: { greater_than_or_equal_to: 0 }
+
+  # Scopes
+  scope :active, -> { where(active: true) }
+  scope :expired, -> { where("expires_at < ? OR active = ?", Time.current, false) }
+  scope :valid, -> { active.where("expires_at > ? OR expires_at IS NULL", Time.current) }
+
+  # Callbacks
+  before_create :increment_uses_count
+
+  # Associations
+  has_many :order_promotion_codes
+  has_many :orders, through: :order_promotion_codes
+
+  private
+
+  def increment_uses_count
+    self.uses_count ||= 0
+  end
+end