Adjusts the on-hand quantity of a stock record up or down (unit_quantity is a signed delta) with a typed reason. Serialized variants take serials instead of a free quantity; batch products take batches. adjustment_type must be one of the company's configured stock-adjustment types (GET /ui/types/stock-adjustment). Requires the Create Stock Adjustment permission on the stock's location. Back-dating via date requires the company backdate setting and is limited to 30 days.