Update stock levels of on-demand items

We weren't bothering with stock when items were on demand anyway. But we
want to track stock now so that we can backorder more when local stock
levels become negative.

app/models/concerns/variant_stock.rb

@@ -112,8 +112,7 @@ module VariantStock
   #
   # This enables us to override this behaviour for variant overrides
   def move(quantity, originator = nil)
-    # Don't change variant stock if variant is on_demand or has been deleted
-    return if on_demand || deleted_at
+    return if deleted_at
 
     raise_error_if_no_stock_item_available
 

lib/open_food_network/scope_variant_to_hub.rb

@@ -43,11 +43,7 @@ module OpenFoodNetwork
       #   - updates variant_override.count_on_hand
       #   - does not create stock_movement
       #   - does not update stock_item.count_on_hand
-      # If it is a variant override with on_demand:
-      #   - don't change stock or call super (super would change the variant's stock)
       def move(quantity, originator = nil)
-        return if @variant_override&.on_demand
-
         if @variant_override&.stock_overridden?
           @variant_override.move_stock! quantity
         else

spec/lib/open_food_network/scope_variant_to_hub_spec.rb

@@ -181,9 +181,9 @@ module OpenFoodNetwork
             scoper.scope v2
           end
 
-          it "doesn't reduce variant's stock" do
+          it "does reduce variant's stock" do
             v2.move(-2)
-            expect(Spree::Variant.find(v2.id).on_hand).to eq 5
+            expect(Spree::Variant.find(v2.id).on_hand).to eq 3
           end
         end
 

spec/models/spree/line_item_spec.rb

@@ -303,8 +303,8 @@ module Spree
           expect(order.shipment.manifest.first.variant).to eq line_item.variant
         end
 
-        it "does not reduce the variant's stock level" do
-          expect(variant_on_demand.reload.on_hand).to eq 1
+        it "reduces the variant's stock level" do
+          expect(variant_on_demand.reload.on_hand).to eq(-9)
         end
 
         it "does not mark inventory units as backorderd" do

spec/system/consumer/shopping/variant_overrides_spec.rb

@@ -210,12 +210,12 @@ RSpec.describe "shopping with variant overrides defined" do
       expect(product1_variant1_override.reload.count_on_hand).to be_nil
     end
 
-    it "does not subtract stock from variants where the override has on_demand: true" do
+    it "does subtract stock from variants where the override has on_demand: true" do
       click_add_to_cart product4_variant1, 2
       click_checkout
       expect do
         complete_checkout
-      end.to change { product4_variant1.reload.on_hand }.by(0)
+      end.to change { product4_variant1.reload.on_hand }.by(-2)
       expect(product4_variant1_override.reload.count_on_hand).to be_nil
     end