The periodic camera cleanup task scans /proc for ffmpeg processes and
kills any not in the active-streams registry. The Obico detection
service's capture_camera_frame_bytes() spawns short-lived ffmpeg for
snapshots but never registered the PID — so cleanup killed it as
"orphaned" mid-capture (SIGKILL, exit -9), producing false errors and
missed detection frames.
Track capture PIDs in _active_capture_pids and exclude them from the
cleanup kill list.
