s_scan_sent (one-shot per boot) becomes s_last_scan_ms; RepeatersScreen
onEnter() re-runs the discover if there has never been a scan or the
last one is older than REPEATER_RESCAN_AFTER_MS (60s). Walking away
and returning now gives a fresh list instead of stale results.
Also fixes the manual KEY_ENTER_LONG rescan: after Phase C removed
the poll() that re-checked the flag, just unsetting it no longer
triggered anything. Now it calls doScan() directly.