Home screen returns 500

At 21:30 your video app's home screen fails to load for everyone, returning a 500. Dashboards: the personalization service (which supplies the recommended rows) has been erroring at 100% since 21:28 after a deploy; your home-screen API calls personalization synchronously and propagates its error instead of degrading; everything else (playback, search, account) is healthy. The home-screen API itself wasn't deployed. How do you triage and mitigate?