Trigger new backorder only when user checks out

When an admin creates an order, then AmendBackorderJob is called which
can also trigger a new backorder if needed.

This means that we are not creating backorders via subscriptions any
more. It has never been requested and we can bring that back if needed.

app/controllers/checkout_controller.rb

@@ -92,6 +92,7 @@ def confirm_order
     end
     @order.process_payments!
     @order.confirm!
+    BackorderJob.check_stock(@order)
     order_completion_reset @order
   end
 

app/models/spree/order.rb

@@ -392,8 +392,6 @@ def finalize!
 
       deliver_order_confirmation_email
 
-      BackorderJob.check_stock(self)
-
       state_changes.create(
         previous_state: 'cart',
         next_state: 'complete',

spec/system/consumer/checkout/backorder_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "system_helper"
+
+RSpec.describe "Checkout" do
+  include ShopWorkflow
+  include CheckoutHelper
+
+  let(:variant) { order.variants.first }
+  let(:order) { create(:order_ready_for_confirmation) }
+
+  before do
+    variant.semantic_links << SemanticLink.new(semantic_id: "https://product")
+    set_order order
+    login_as create(:user)
+  end
+
+  it "triggers a backorder" do
+    visit checkout_step_path(:summary)
+
+    expect { place_order }.to enqueue_job BackorderJob
+  end
+end