Checks for existing proxies are not reliable

[NathanLovato]

When checking for proxies that need to be rebuilt, BPSProxy (ffprobe) treats some existing proxies as being outdated even if the source video didn't change, leading to unnecessary transcoding operations.

With the long videos I deal with, I end up having to place new recordings in temporary directories, generating proxies there, before moving files back to the footage directory.

We need a more reliable system. As there doesn't seem a reliable parameter to map the generated proxy to the source video, how about writing some json file to the generated proxy directory with the last modification date for each source video file?