SD2 (14 §3): out of credits pauses IPQualityScore (one warning, one
event), the FTC list answers meanwhile, the dashboard shows a dismissable
spam_quota warning, and spamQuotaCheck resumes it every 6 hours once
credits are back. A newly saved key resumes it at once.
Follow-up: confirm with the owner's real key that the Credit Usage API
costs no credit (TODO in 14 §2.1).