Add foreign key constraint to Spree::OrderPromotions table

This adds a foreign key constraint that makes sure at the database level
that we don't have non-existent order IDs in the spree_orders_promotions
table.

core/db/migrate/20231027084517_add_order_promotions_foreign_key.rb

@@ -0,0 +1,10 @@
+class AddOrderPromotionsForeignKey < ActiveRecord::Migration[7.0]
+  def up
+    Spree::OrderPromotion.left_joins(:order).where(spree_orders: { id: nil }).delete_all
+    add_foreign_key :spree_orders_promotions, :spree_orders, column: :order_id, validate: false, on_delete: :cascade
+  end
+
+  def down
+    remove_foreign_key :spree_orders_promotions, :spree_orders
+  end
+end