W149: Add item to sales order

Appends a new line to an existing SO. Triggered from Sales Order Detail items section.

Steps

1. Load the SO. Call SalesOrders.getSoById to confirm existence.

2. Validate the SKU. Call Products.getSkuById to confirm it exists.

3. Insert the line. Call SalesOrders.addItem with quantity and unit price. A unique violation on the L1 UNIQUE (sales_order_id, sku_id) constraint (detected via isUniqueViolation) is mapped to ConflictError — one line per SKU per order.

Returns

The new line.

Business rules

• One line per SKU per order. Duplicate SKUs on the same SO are rejected with a 409 conflict, enforced at L1.
• No ledger side effects in v1. FG-ledger DEMAND / ALLOCATE entries for item changes after create are deferred; the initial entries are written by W144.

Errors

• NotFoundError. The SO or SKU was not found.
• ConflictError. The SKU is already on this sales order.