Debian Forky's stock apache2 systemd unit sets PrivateDevices=yes,
giving Apache its own /dev namespace without video devices. This
caused file_exists('/dev/video0') to return false even though the
device exists and www-data is in the video group.
Remove the file_exists() pre-check and just attempt v4l2-ctl directly.
When it fails, provide targeted diagnostics: missing v4l2-ctl, systemd
PrivateDevices detected, or generic permission error with the running
username.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
