Only allow upgrades for timestamps where the cam was on stream

[changesbyjames]

We want to retain the privacy of the live cams so we should explicitly only allow upgrades for periods where the requested cam was active.

This may need integration with the chatbot & keeping a log of what was in each slot at any given time. We also might not need that if the streams are being pull on-demand because it might be that mediamtx doesn't record anyway in those circumstances. We'll need to experiement!

[linear]

ALV-15 Only allow upgrades for timestamps where the cam was on stream

We want to retain the privacy of the live cams so we should explicitly only allow upgrades for periods where the requested cam was active.

This may need integration with the chatbot & keeping a log of what was in each slot at any given time. We also might not need that if the streams are being pull on-demand because it might be that mediamtx doesn't record anyway in those circumstances. We'll need to experiement!