Add KEEP_LAST retention policy for backups

Add KEEP_LAST environment variable to always preserve the N most recent
backups regardless of age, preventing data loss when backups haven't
been created for extended periods. Defaults to 10 copies.

scripts/deleteold.sh

@@ -2,14 +2,38 @@
 
 set -u
 
-# Seconds since epoch for current time
+# Retention: delete backups older than DELETE_AFTER days, but always keep
+# at least KEEP_LAST most recent copies regardless of their age.
 DATE_NOW=$(date +%s)
 DEST="${MINIO_PATH%/}/"
+KEEP_LAST="${KEEP_LAST:-10}"
 
-/scripts/minio_uploader.sh list "$DEST" | grep "sqlite_" | while read -r LINE
+case "$KEEP_LAST" in
+  ''|*[!0-9]*) echo "WARN: KEEP_LAST is not a number ('$KEEP_LAST'), falling back to 10"; KEEP_LAST=10 ;;
+esac
+
+LIST=$(mktemp)
+trap 'rm -f "$LIST"' EXIT
+
+# `rclone ls` prints "<size> <path>". Filenames are sqlite_<YYYY-MM-DD-HHMMSS>.tar.gz,
+# so a reverse lexicographic sort is a reverse chronological sort.
+/scripts/minio_uploader.sh list "$DEST" \
+  | awk '{ print $2 }' \
+  | grep "^sqlite_" \
+  | sort -r > "$LIST"
+
+TOTAL=$(wc -l < "$LIST" | tr -d ' ')
+echo "Found $TOTAL backups, keeping at least $KEEP_LAST most recent, max age $DELETE_AFTER days"
+
+if [ "$TOTAL" -le "$KEEP_LAST" ]; then
+  echo "Nothing to rotate: $TOTAL backups <= KEEP_LAST=$KEEP_LAST"
+  exit 0
+fi
+
+# Only the tail beyond the protected window is a candidate for deletion.
+tail -n "+$((KEEP_LAST + 1))" "$LIST" | while read -r BACKUP_FILENAME
 do
-  # `rclone ls` prints "<size> <path>"
-  BACKUP_FILENAME=$(echo "$LINE" | awk '{ print $2 }')
+  [ -n "$BACKUP_FILENAME" ] || continue
 
   BACKUP_DATE=$(echo "$BACKUP_FILENAME" | awk 'BEGIN { FS = "[_-]" } ; { printf "%s-%s-%s",$2,$3,$4 }')