add a thread check (with config for how long after) to check ongoing recording for "No signal"

[yarikoptic]

As observed with /data/reprostim/Videos/2025/09/2025.09.08-13.30.25.317--.mkv we are recording huge "No signal file". We should add a setting for "check_delay" to

• run background process for no_signal check on currently captured file
  • run with nice 10 or so to make it the lowest priority
• if continuous "no signal" -- we should terminate the ffmpeg process, cleanup the "no signal video" and do not start new one until
• ...