gitea-backup-pull: switch vlda-01 retention from a fixed count to a 30-day rotation

KEEP=30 (last 30 archives) approximated 30 days only if there was
exactly one dump per day. Prune by file mtime instead (find -mtime +30)
so a catch-up burst after time offline doesn't prematurely evict
recent dumps, and a quiet stretch doesn't hoard month-old ones.

scripts/clients/vlda-01/user-scripts/gitea-backup-pull/script

@@ -28,7 +28,7 @@ REMOTE_PORT=10022
 UD_LABEL=WSD2L840
 UD_MOUNTPOINT=/mnt/disks/WSD2L840
 LOCAL_DIR="$UD_MOUNTPOINT/git"
-KEEP=30
+KEEP_DAYS=30
 
 # WSD2L840 is an Unassigned Device, not part of the array - not
 # guaranteed to already be mounted when this runs. Resolve by label
@@ -150,11 +150,17 @@ if [ -s "$DEL_BATCH" ]; then
 	fi
 fi
 
-# prune to the last $KEEP archives (vlda-01 has far more headroom than
-# alpha, so this is the real long-term history now that alpha deletes
-# its own copies right after a verified pull)
+# prune anything older than $KEEP_DAYS calendar days (vlda-01 has far
+# more headroom than alpha, so this is the real long-term history now
+# that alpha deletes its own copies right after a verified pull). A
+# calendar-day window rather than a fixed count on purpose: a burst of
+# catch-up pulls after being offline a while shouldn't prematurely evict
+# genuinely recent dumps just because there are suddenly more than a
+# fixed count, and a quiet stretch with few new dumps shouldn't keep
+# month-old ones around just because too few newer ones have arrived to
+# push them out.
 cd "$LOCAL_DIR" || exit 1
-ls -1t gitea-dump-*.zip 2>/dev/null | tail -n +$((KEEP + 1)) | while read -r old; do
+find . -maxdepth 1 -name 'gitea-dump-*.zip' -mtime +"$KEEP_DAYS" -print0 | while IFS= read -r -d '' old; do
 	rm -f "$old" "${old}.sha256"
 done