Validate and assess a selfie via session (mobile)

Per-step selfie validation for the cross-device flow. Always checks that a face is present. When the request also carries the liveness capture (liveness_frame files and the liveness_token field), it runs the selfie assessment now (liveness, age estimation, and the face match against the document already processed by validate-document) and the session carries the DECISION, never the selfie or the frames, to submit, which then needs no images. assessed: false means the assessment did not run (no capture sent, or nothing to match against) and the submit must carry the selfie and liveness as before. Free, no credits consumed.