Add Kopia backup stack with Backblaze B2 offsite target.

Protects /config, selective labdata, and nightly DB dumps; documents homelab improvement plan briefs.

stacks/backup/scripts/backup-run-snapshots.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+set -eu
+
+[ -f /etc/backup.env ] && . /etc/backup.env
+
+NTFY_URL="${NTFY_URL:-https://ntfy.ginnoir.com/backup}"
+NTFY_TITLE="${NTFY_TITLE:-ValhallaBackup}"
+
+notify() {
+  priority="$1"
+  message="$2"
+  curl -sf -H "Title: ${NTFY_TITLE}" -H "Priority: ${priority}" -d "${message}" "${NTFY_URL}" >/dev/null 2>&1 || true
+}
+
+if ! docker ps --format '{{.Names}}' | grep -qx kopia; then
+  notify high "Kopia container not running — snapshots skipped"
+  exit 1
+fi
+
+FAILED=0
+while IFS= read -r path || [ -n "${path}" ]; do
+  case "${path}" in ''|\#*) continue ;; esac
+  if ! docker exec kopia kopia snapshot create "${path}"; then
+    FAILED=1
+  fi
+done < /snapshot-paths.txt
+
+if [ "${FAILED}" -eq 0 ]; then
+  notify default "Snapshots OK ($(date +%Y-%m-%d\ %H:%M))"
+else
+  notify high "One or more snapshots FAILED — check docker logs kopia"
+  exit 1
+fi