Add playback stats endpoint similar to Invidious's /api/v1/stats

[syeopite]

It should be possible to track 403 videoplayback attempts and provide it to Invidious.

When combined with additional tracking in Invidious for YouTube's "this content is not available" or "this helps protect our community" message alongside the existing playback success rate tracking it should be possible to paint a more accurate picture for whether the instance is blocked.