chore: add migration helper scripts + cutover plan doc

R510 -> R730XD/Proxmox migration helpers (consistent DB dumps, B2/Kopia snapshot refresh, cutover final backup) plus the previously-untracked gitea-portainer-registry cutover plan under docs/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/refresh-b2-snapshot.sh

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+#!/usr/bin/env bash
+#
+# Pre-migration: make the Kopia -> B2 snapshots current AND complete.
+#
+# 1) waits for the running migration-backup.sh to finish (avoid /config read contention)
+# 2) supplements the logical DB dumps the nightly job omits (nextcloud, immich) and
+#    replaces the empty bookstack dump, using the complete set from migration-backup.sh
+# 3) runs the existing Kopia snapshot pipeline -> fresh B2 snapshot of /config + dumps + labdata
+#
+# Run ON valhalla as ginnoir:  nohup bash ~/refresh-b2-snapshot.sh > ~/refresh-b2-snapshot.out 2>&1 &
+#
+set -uo pipefail
+log(){ printf '[%s] %s\n' "$(date +%H:%M:%S)" "$*"; }
+
+# 1. wait for the migration backup to complete
+log "waiting for migration-backup.sh to finish..."
+while pgrep -f 'migration-backup.sh' >/dev/null 2>&1; do sleep 15; done
+log "migration backup finished"
+
+# locate newest migration backup db dir and newest official dump dir
+MIG_DB="$(ls -1dt /storage1/migration-backup/*/db 2>/dev/null | head -1)"
+DUMPS="$(ls -1dt /config/backup/dumps/*/ 2>/dev/null | head -1)"
+log "migration db dir: ${MIG_DB:-<none>}"
+log "official dump dir: ${DUMPS:-<none>}"
+
+# 2. supplement the dumps the nightly job misses (raw datadirs already cover them in /config,
+#    but this gives B2 a complete logical-dump layer too)
+if [ -n "${MIG_DB:-}" ] && [ -n "${DUMPS:-}" ]; then
+  for f in nextcloud-postgres.sql.gz immich-postgres.sql.gz mariadb_bookstack.sql.gz; do
+    if [ -s "$MIG_DB/$f" ]; then
+      sudo cp -f "$MIG_DB/$f" "$DUMPS/$f" && log "supplemented $f ($(du -h "$DUMPS/$f" | cut -f1))"
+    else
+      log "WARN: $MIG_DB/$f missing/empty — not supplemented"
+    fi
+  done
+  log "dump dir now:"; ls -lh "$DUMPS"
+fi
+
+# 3. fresh Kopia -> B2 snapshot (same pipeline cron uses)
+log "running Kopia snapshot to B2..."
+docker exec backup_scheduler sh /scripts/backup-run-snapshots.sh 2>&1
+
+log "DONE — verify with: docker exec kopia sh -c 'KOPIA_PASSWORD=\$KOPIA_REPOSITORY_PASSWORD kopia snapshot list --all' | tail"