P5: redbear-polkit now enforces real authorization:
- is_authorized(uid, action_id) checks UID-based policy
- uid=0 (root) always authorized
- Other users checked against /etc/polkit-1/policy.toml
- Default: deny for unknown actions (fail-closed)
- Backend name changed from 'redbear-permit-all' to 'redbear-uid-policy'
- Default policy grants power/network/storage to root+user(1000)
