enable actions for multi (group) orders and ordergroup invoices

db/migrate/20250515111627_add_multi_group_order_id_to_ordergroup_invoices.rb

@@ -0,0 +1,5 @@
+class AddMultiGroupOrderIdToOrdergroupInvoices < ActiveRecord::Migration[7.0]
+  def change
+    add_reference :ordergroup_invoices, :multi_group_order, foreign_key: true, type: :integer
+  end
+end

db/migrate/20250516104953_make_group_order_key_nullable.rb

@@ -0,0 +1,12 @@
+class MakeGroupOrderKeyNullable < ActiveRecord::Migration[7.0]
+  def change
+    # Make column nullable (safe even if already is)
+    change_column_null :group_orders, :multi_group_order_id, true
+
+    # Remove old FK if it exists (avoid name conflict)
+    remove_foreign_key :group_orders, column: :multi_group_order_id
+
+    # Re-add with ON DELETE SET NULL
+    add_foreign_key :group_orders, :multi_group_orders, column: :multi_group_order_id, on_delete: :nullify
+  end
+end