Reads which digest an emailed unsubscribe link turns off, so that the
/unsubscribe page can name it before the reader confirms. The server
checks the token's signature when the page posts it; this only reads it.
f
readUnsubscribeCadence(token: string): DigestCadence | null
Returns the cadence token names, or null when it isn't an unsubscribe
token at all.
Usage
import * as mod from "src/utils/auth/unsubscribe.ts";