The probe fired the instant a batch finished, but a fresh drop takes ~1-2 min to
appear even with the server's auto-scan ON — so it always missed and we crawled
anyway, defeating the optimization. Now probe_present_libraries POLLS each candidate
over a grace window (probe_grace_minutes, default 2), skipping a library's crawl as
soon as the server reports it has the item, and only crawling what's still missing
when grace expires. The probe target for a media type you DIDN'T just download is an
old item the server already has → confirmed instantly, no wait. grace=0 probes once.
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