Create an order

Creates one order with its lines. Client and location are referenced by code. status is planned for orders that are known but not yet actionable (tomorrow's work), pending for orders to wave now. Lines are the SKUs on the order; when present they set the unit and line counts. Fails if the order number already exists; use POST /orders/batch for idempotent re-sends. Wave business rules run on every API create and may override status or priority or mark the order to skip automatic waving.