Generate Shipment Packing Slip

Body for POST /admin/projects/{id}/packing-slip.

All fields are optional. The endpoint is idempotent — calling it a second time updates the override values on the existing row and re-renders the PDF (preserving packing_slip_number).

Carrier + tracking are intentionally NOT included here — they live on the Shipment row and are surfaced via the existing "Create / Edit Shipment" admin flow. The packing slip PDF reads them at render time when a Shipment exists.

202 Accepted response from POST .../packing-slip.

The actual PDF render runs asynchronously in the Temporal worker. Frontend re-fetches the parent detail query to surface the new packing_slip_pdf_url.