Cache busting attack

Your product-search API origin CPU saturates and p99 triples, but your CDN cache-hit ratio quietly collapsed from 96% to 11% over an hour. Traffic volume at the edge is only up ~20%, not a volumetric flood. Inspection shows requests appending random query params (?_=<rand>, &v=<uuid>) to otherwise cacheable search URLs, busting the cache so every request hits origin. Sources are ~40k residential IPs, realistic UAs, each slow and human-paced; many requests carry a valid logged-in cookie. It correlates with a competitor's price-scraping season. How do you triage and mitigate without blocking real shoppers?