serveEntity now schedules a Bump recheck for an entity whose art was recorded
absent, so viewing a missing cover re-triggers resolution (e.g. after an
external source that was down during the scan comes back), matching the
request-time bump that already covers never-resolved entities.
Throttled by attempted_at against requestRecheckAge (1h) so repeatedly opening
a genuinely-absent page can't hammer external services. EnqueueBump preserves
an existing failed-state backoff via MAX(priority,...) and inserts a fresh,
immediately-eligible recheck for a settled-absent row (whose queue row was
already deleted).
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