Calculate stock from DFC wholesale variants

[mkllnk]

What? Why?

• Closes [DFC Orders] Compute stock of retail variants from wholesale variants #12928

When importing DFC products, two products can be coupled as retail variant and wholesale variant of the same product. We aim to sell the retail variant through an OFN shop and then place backorders for the wholesale variant to make use of economies of scale.

We already calculated the price of a retail variant from its wholesale variant:

• DFC Orders: Calculate wholesale price for imported retail variants #13033

Now we also calculate the stock level from the wholesale variant.

What should we test?

• Import a DFC catalog at Admin -> Product Import
• Observe the product prices and stock levels.
• Add retail products to an order cycle.
• Change stock levels in Shopify.
• Add a retail variant to your OFN cart.
• The stock level should get updated correctly.

Release notes

Changelog Category (reviewers may add a label for the release notes):

• User facing changes
• API changes (V0, V1, DFC or Webhook)
• Technical changes only
• Feature toggled

The title of the pull request will be included in the release notes.

Dependencies

Documentation updates