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?

db/migrate/20250928180837_create_promotion_codes.rb

@@ -7,8 +7,12 @@ class CreatePromotionCodes < ActiveRecord::Migration[8.0]
       t.boolean :active, default: true, null: false
       t.integer :usage_limit, default: nil
       t.integer :uses_count, default: 0, null: false
-      t.datetime :created_at, null: false
-      t.datetime :updated_at, null: false
+
+      # Reference user(promoter) who has created the promotion code
+      t.references :user, null: false, foreign_key: true
+      t.references :event, null: false, foreign_key: true
+
+      t.timestamps
     end
 
     # Unique index for code