Create or update a product

Products are upserted by external_id within the business store: an existing product with the same external_id is updated, otherwise a new one is created. product_units are merged by unit external_id: matching units are updated in place (keeping their IDs and video associations), new units are added, and units omitted from the list are left untouched — removing a unit requires the unit DELETE endpoint. When updating an existing product, every sent unit must include an external_id (422 otherwise). For simple products without variants, the product-level unit fields (sku, gtin, mpn, barcode, price, original_price, quantity, url) apply to the product's single unit instead of product_units; they are rejected when combined with product_units or when the product has multiple units.