admit_configured_roots() re-probed every configured root on every scan
(os.path.exists + realpath per root), so even a single-root refresh
statted every configured drive first — enough to spin up sleeping disks
and stall the scan the user actually asked for.
Admission only ever appends, so roots already in the live lists can
never gain entries from re-preparation. Probe only the configured roots
that are not live yet; the plug-in-a-drive-later behavior is unchanged
(a missing root is re-probed on each call until admitted).