Create attachment

Create a draft upload URL for a shipment attachment.

This is step 1 of the shipment attachment workflow.

Workflow:

1. Request an upload URL for a specific order + shipment.
2. Upload image bytes directly.
3. Finalize the attachment in a second call.

Rules:

• Shipment must belong to an order owned by the authenticated seller.
• Attachments are only allowed within a limited time window after shipment creation.
• Returns 403 when attachment uploads are no longer allowed.