Debug API Errors in Production

You're the on-call engineer at Instasnap, a photo-sharing app with about 3 million daily active users. At 14:09 UTC you get paged: `[PAGE] api-service error rate 42% (threshold 2%) - 5xx elevated across all endpoints` Support tickets are flooding in. Photos won't post, the upload spinner runs forever, and some users say the whole app is broken: feeds won't load and a few can't even log in. As far as you know, nobody has touched anything. You have full access to logs, metrics, dashboards, and the shell. I'll play every tool you use. Tell me what you'd look at or run, and I'll tell you what you see. Where do you start?