Authd management protocol extension

[vhrabar]

Description

Detection runs only in Python, and the camera can only be held by one process, so live enrollment and testing must be daemon-side sessions the GUI subscribes to over the socket. Extend the framed protocol with a mana side.

Tasks

• Status messages carry: face present/absent/multiple, bounding box, brightness/dark_threshold flag, match distance vs certainty, timing
• frame streaming -> daemon sends downscaled JPEG frames over the socket; real preview, bounded size/rate mandatory
• Authorization on management messages: peer-credential (SO_PEERCRED) checks, a user may only manage their own models; root/admin path defined
• Daemon-side session implementation reusing auth/session.py, recorders/video_capture.py, auth/model_store.py
• Strict timeouts; a dead GUI client must abort the session and release the camera

Acceptance criteria

• PAM auth requests are unaffected while a management session runs (or are cleanly prioritized — documented either way)
• Killing the GUI mid-enrollment releases the camera within the protocol timeout