Initial planning docs and hardening scripts for HA rebuild

Covers node 1 hardware/network layout, LVM-thin -> ZFS migration path,
Ceph as the future HA storage upgrade, and baseline SSH/firewall
hardening.

scripts/harden-ssh.sh

@@ -0,0 +1,51 @@
+#!/bin/bash
+# Apply baseline SSH hardening to a Proxmox VE node: key-only root login
+# + fail2ban. Idempotent - safe to re-run. Run as root on the PVE host.
+set -euo pipefail
+
+DROPIN_DIR="/etc/ssh/sshd_config.d"
+DROPIN_FILE="${DROPIN_DIR}/99-hardening.conf"
+
+if [ "$(id -u)" -ne 0 ]; then
+    echo "Must run as root." >&2
+    exit 1
+fi
+
+mkdir -p "$DROPIN_DIR"
+cat > "$DROPIN_FILE" <<'EOF'
+PermitRootLogin prohibit-password
+PasswordAuthentication no
+EOF
+echo "Wrote $DROPIN_FILE"
+
+if ! authorized_keys_present=$(find /root/.ssh/authorized_keys /home/*/.ssh/authorized_keys -type f 2>/dev/null | head -n1); then
+    authorized_keys_present=""
+fi
+if [ -z "$authorized_keys_present" ]; then
+    echo "WARNING: no authorized_keys found for any user yet." >&2
+    echo "Add your SSH public key before disconnecting, or you'll lock yourself out." >&2
+fi
+
+sshd -t
+systemctl reload sshd
+echo "sshd reloaded with key-only root login."
+
+if ! dpkg -s fail2ban >/dev/null 2>&1; then
+    apt-get update
+    apt-get install -y fail2ban
+fi
+
+mkdir -p /etc/fail2ban/jail.d
+cat > /etc/fail2ban/jail.d/sshd.local <<'EOF'
+[sshd]
+enabled = true
+port = ssh
+backend = systemd
+maxretry = 5
+bantime = 1h
+findtime = 10m
+EOF
+
+systemctl enable --now fail2ban
+systemctl restart fail2ban
+echo "fail2ban enabled for sshd."