Finalize cover upload

Finalize a previously uploaded cover image and apply it to the authenticated user account.

This endpoint completes the cover upload process after the binary upload has finished.

Finalization behavior:

• Validates that the uploaded image identifier is valid for the current user.
• Confirms upload completion.
• Performs content checks before applying the image.
• Returns a rejection response if the uploaded content is not allowed.

Typical sequence:

1. POST /user/account/cover
2. Upload image to returned URL.
3. PATCH /user/account/cover, which is this endpoint.