Create attachment

Create a draft attachment upload for a listing and return a direct upload URL.

This is step 1 of the listing attachment workflow.

Workflow:

1. Call this endpoint to obtain id and uploadURL.
2. Upload the image bytes directly to the returned URL.
3. Call finalize attachment to validate and persist the attachment on the listing.

Rules:

• Listing must belong to the authenticated seller.
• Listing must be active (quantity > 0).
• The returned image identifier is namespaced to the listing and seller context.

Response:

• Returns an ImageUpload payload with the image identifier and direct upload URL.