Create case attachment

Create a draft upload URL for a seller case attachment.

This is step 1 of the case attachment flow.

Workflow:

1. Call this endpoint to receive an image id and direct uploadURL.
2. Upload image bytes directly to the returned URL.
3. Finalize the upload with the finalize attachment endpoint.

Rules:

• Case must exist and belong to one of the seller’s orders.
• Non-owned or missing cases return 404.