Guard arr cache writes against in-flight rescans, add rescan monitor tool

A direct arr_cache_write() call mid-rescan wrote a partial snapshot that
looked like real data loss to every consumer of the cache. The guard now
lives in arr_cache_write() itself so every caller is protected, not just
arr_get_tracked_data(). arr_rescan_monitor.sh closes the resulting gap for
rescans triggered outside arr_full_rescan.sh's own trigger-and-wait path.

Arrs_Stack/arr_full_rescan.sh

@@ -86,7 +86,7 @@ for arr in lidarr sonarr radarr; do
 
     active=$(arr_active_rescan_command "$arr" "$url" "$key" "$ver")
     if [[ -n "$active" ]]; then
-        warn "${arr^} already mid-rescan ($active) — skipping, will catch it next scheduled run"
+        warn "${arr^} already mid-rescan ($active) — skipping, will catch it next scheduled run (run Tools/arr_rescan_monitor.sh ${arr} to refresh its cache as soon as this one finishes instead of waiting)"
         (( SKIPPED++ ))
         continue
     fi