Uses the unguessable capture-session ID from the QR handoff. The session must
be pending and unexpired. Face photos use the face quality check; body photos
use the same body check as /skin/validate-body-images, before files are stored
or the session is marked images_uploaded. A rejected upload leaves the session
pending for a retake. In a multi-area handoff, keep already accepted sessions
and retry only the remaining areas. No scan analysis or payment is started here.