Disconnect preview image generation (timeline scrubbing) from recording config

[DrieWielRr]

Because preview generation is tied to the recording config when I want 48h of scrubbing view but don't want 48 of recording data on disk because that would only cause useless storage usage and extra wear and tear on the disks having to keep writing the 10s segments from RAM to disk instead of writing a small image file every x seconds.
Currently there is no way to do this in the because the preview image generation is hardcoded in to the recording setting.

I'd like to see something like:
record:
preview:
quality: very_high
retain:
days: 1 (or in hours, like hours: 8)

This way frigate can generate the preview timeline so its fully filled and not having to put on the black "no preview found" image when a user is scrubbing through it (the "no preview found" also makes it looks like the system was broken during that time)..
This also ensures that when a camera misses a motion event for whatever reason I still have a general understanding of what happened in that situation based on the preview timeline.