Feature: Add 'idle time' metric

### What feature or improvement do you think would benefit the app?

This isn't *necessarily* an admin feature, but it would be interesting to add a feature that allows measuring the amount of time in total a user spent 'idle' in a timelapse.
It would work by calculating the difference between each frame in the video, and totalling the amount of frames where the next frame was identical to the original.

### Requirements

- [ ] Timelapse API implemented
- [ ] Client implementation for 'video diff' (ffmpeg?)

### Version

_No response_

### API Changes

_No response_

### Intended Use-Case

This would make it easier to review timelapses and as just a general statistic to make the app more interesting.

### Comments

Because it is calculated client-side, this could only be a feature on desktop.