feat(migrations): Add promotion code migrations

db/migrate/20250928180837_create_promotion_codes.rb

@@ -1,7 +1,7 @@
 class CreatePromotionCodes < ActiveRecord::Migration[8.0]
   def change
     create_table :promotion_codes do |t|
-      t.string :code, null: false, unique: true
+      t.string :code, null: false
       t.integer :discount_amount_cents, null: false, default: 0
       t.datetime :expires_at
       t.boolean :active, default: true, null: false
@@ -11,6 +11,7 @@ class CreatePromotionCodes < ActiveRecord::Migration[8.0]
       t.datetime :updated_at, null: false
     end
 
+    # Unique index for code
     add_index :promotion_codes, :code, unique: true
   end
 end