feat(installer): grant brightness control to the video group

Arch's brightnessctl ships no udev rules and leans on logind, which grants brightness writes only to the active seat session. Anything outside that session (a script, an ssh shell, a panel launched into a different one) meets root-owned sysfs and gets EPERM. On velox both panel sliders were dead after a fresh install.

configure_backlight_access writes a udev rule making the backlight and keyboard-LED brightness attributes group-writable by video, which create_user already adds the user to. I granted the keyboard LED to video rather than input: anyone in input can read every input device, which is too much authority for dimming a keyboard.

essential_services is already marked complete on machines installed before this step, so a re-run skips them. The post-install checklist carries the manual route.

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