Add Purge Threshold

Adds a purge threshold setting. This setting allows the user to
control at what filled percentage the purge activities are run.
This replaces the default, hard coded 95% point.

The purge threshold defaults at the original 95%. I set a minimum
of 20 and max of 99 because those values felt sensible but am open
to changing those based on feedback.

Note: The purge threshold is still active when the keep option is
set. I added a note for this but this still presents some risk
where users who change this while in Keep mode could have their
services shut down earlier than they expect.

scripts/disk_check.sh

@@ -3,8 +3,9 @@ set -x
 
 source /etc/birdnet/birdnet.conf
 used="$(df -h ${EXTRACTED} | tail -n1 | awk '{print $5}')"
+purge_threshold="${PURGE_THRESHOLD:-95}"
 
-if [ "${used//%}" -ge 95 ]; then
+if [ "${used//%}" -ge "$purge_threshold" ]; then
 
   case $FULL_DISK in
     purge) echo "Removing oldest data"
@@ -34,7 +35,7 @@ if [ "${used//%}" -ge 95 ]; then
   esac
 fi
 sleep 1
-if [ "${used//%}" -ge 95 ]; then
+if [ "${used//%}" -ge "$purge_threshold" ]; then
   case $FULL_DISK in
     purge) echo "Removing more data"
        rm -rfv ${PROCESSED}/*;;