Ops: add Proxmox migration tooling and runbook

Add end-to-end migration scripts for inventory, precopy, cutover, smoke tests, rollback, and post-migration checks. Include an operational runbook and Proxmox env template to move Hördle behind Nginx Proxy Manager while preserving persistent volumes safely.

scripts/migration-rollback.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+# Rollback-Hilfsskript: stoppt Ziel-Container und zeigt klare Operator-Hinweise.
+# Die eigentliche Rückleitung des Traffics erfolgt im Nginx Proxy Manager.
+
+TARGET_HOST="${TARGET_HOST:-root@10.0.0.19}"
+TARGET_APP_DIR="${TARGET_APP_DIR:-/opt/hoerdle}"
+TARGET_COMPOSE_FILE="${TARGET_COMPOSE_FILE:-docker-compose.yml}"
+SSH_OPTS="${SSH_OPTS:-}"
+
+echo "== Hördle Rollback-Hilfe =="
+echo "Zielhost: $TARGET_HOST"
+echo
+
+echo "-- Ziel-App stoppen --"
+ssh $SSH_OPTS "$TARGET_HOST" "cd '$TARGET_APP_DIR' && docker compose -f '$TARGET_COMPOSE_FILE' down"
+echo
+
+echo "Naechste Schritte:"
+echo "1) NPM Proxy Host sofort wieder auf alte VPS-Instanz umstellen."
+echo "2) Externen Smoke-Test auf alter Instanz durchfuehren."
+echo "3) Fehleranalyse im Ziel-LXC (docker compose logs)."