Set cost inputs

Insert or update cost inputs in bulk — the endpoint a spreadsheet or CSV import writes through. Rows are keyed on (product_id, sku_id, cost_type, effective_from): re-sending a key overwrites its amount rather than stacking a second cost onto the same units, so replaying the same import is safe. Omit sku_id to set a product-level default that applies to every SKU without one of its own. Requires read_write scope and a single shop.