CDN purge melts origin

After a content-management deploy, a job issued a global CDN purge of the entire site ('purge everything') to be safe. Minutes later, origin RPS spikes ~25x, origin latency and 5xx climb, and the CDN hit ratio drops near zero across all POPs at once as every edge has to re-fetch from a cold cache. Users see slow loads and intermittent errors. The purge was meant to clear a few changed pages, not the whole site. How do you triage the impact, get the site healthy, and prevent a blanket purge from melting origin again?