Finalize attachment

Finalize an uploaded buyer case attachment.

This is step 2 of the buyer case attachment workflow.

Validation:

• imageId must match the expected case attachment URN.
• Upload must be completed (draft uploads are rejected).
• Case must belong to the authenticated user.

Responses:

• 400 for invalid IDs or incomplete uploads.
• 404 for missing/non-owned cases.
• Returns attachment metadata with a display-ready URL.