A full crawl is expensive and a fix can land any day. Sentinels close that gap: a small set of URLs, checked often, compared on a hash of their SEO-relevant fields rather than on their whole HTML.
Hashing only the fields that matter is what makes the signal usable. A page whose HTML changes hourly because of a timestamp or a rotating banner would otherwise report a change every hour and be ignored within a week.
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No card, and the free tier does not expire — the first verdict takes about six weeks to arrive.