Validate and process a document image via session (mobile)

Per-step document validation for the cross-device flow. Checks image quality, then runs the document work (text extraction, face template) so that the submit step does not need the document image again: the session carries the result, never the image, until the session is submitted, cancelled, or expires. Send side (front, back or passport) and document_type as form fields ahead of the file. retained: false means the document was not held (encryption unavailable, or the document could not be processed) and must be included in the submit. Free, no credits consumed.