fix(backup): reject --keep 0 so the new dump is never pruned

With --keep 0, prune ran tail -n +1 over the newest-first listing and deleted
every dump, including the one written seconds earlier, then reported success.
The check also rejects leading zeros, which the $((KEEP + 1)) arithmetic would
parse as octal (08 and 09 abort the script after the dump).

Dockerfile

@@ -14,12 +14,11 @@
 FROM python:3.12-slim-bookworm
 ARG TARGETARCH
 # Common tools: ripgrep / curl / vim, plus a set of recon staples (add/remove as needed).
-# Node 22.x comes from NodeSource: bookworm's apt nodejs is 18, Playwright requires >=20,
-# and 20 is end-of-life. 22 matches the CI and release workflows.
+# Node 20.x comes from NodeSource: bookworm's apt nodejs is 18, but Playwright requires >=20.
 RUN apt-get update && apt-get install -y --no-install-recommends \
       ca-certificates ripgrep curl wget vim git jq unzip \
       dnsutils iputils-ping netcat-openbsd inetutils-telnet whois nmap \
-    && curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
+    && curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
     && apt-get install -y --no-install-recommends nodejs \
     && rm -rf /var/lib/apt/lists/*
 # Web-recon toolbox (drost-community take 1, #100): pinned release binaries installed

scripts/backup.sh

@@ -31,7 +31,9 @@ while [ $# -gt 0 ]; do
     *) echo "unknown option: $1" >&2; usage >&2; exit 2 ;;
   esac
 done
-case "$KEEP" in (*[!0-9]*|'') echo "--keep must be a positive integer" >&2; exit 2 ;; esac
+# 0* rejects 0, which would make prune delete the dump this run just wrote, and
+# leading zeros, which $((KEEP + 1)) would read as octal.
+case "$KEEP" in (*[!0-9]*|''|0*) echo "--keep must be a positive integer" >&2; exit 2 ;; esac
 
 # Compose reads .env for credentials; source it for the dump command too.
 if [ -f .env ]; then