Merge PR #16: chequeo de frescura de backups

/usr/local/bin/cayce-check-backup-freshness (from
deploy/backup/check-freshness.sh): exits 1 if the newest
/var/backups/cayce/YYYY-MM-DD/ dir has mtime older than 26h, or if no
backups exist while the age recipient IS configured (backups expected).
Info-exit-0 when backups have never been configured. Not attached to a
timer (only cayce-backup.timer was authorized); documented for manual
or cron use.

Verified: fresh dir => OK 0h exit 0; dir with 2-day mtime => ALERT 48h
exit 1; no-dir + recipient => ALERT; no-dir + no-recipient => INFO.

deploy/backup/check-freshness.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# check-backup-freshness — alert if the newest backup is older than 26h.
+#
+# Exit 0 = fresh (or no backups ever made and NONE expected yet)
+# Exit 1 = stale: last backup dir older than MAX_HOURS, or no dir at all
+#          while the age recipient is configured (backups should exist).
+#
+# Wire it anywhere you like (cron, another timer, or run manually from the
+# Mac after the daily pull). Not attached to any timer by itself — the only
+# authorized timer in Tanda B is cayce-backup.timer.
+set -euo pipefail
+
+BACKUP_ROOT="/var/backups/cayce"
+MAX_HOURS=26
+RECIPIENT_FILE="/etc/cayce-backup/age-recipient.pub"
+
+newest="$(find "$BACKUP_ROOT" -mindepth 1 -maxdepth 1 -type d -name "2*-*-*" 2>/dev/null | sort | tail -1 || true)"
+
+if [[ -z "$newest" ]]; then
+  if [[ -f "$RECIPIENT_FILE" ]]; then
+    echo "ALERT: no backups exist although the age recipient is configured" >&2
+    exit 1
+  fi
+  echo "INFO: no backups yet (age recipient not configured) — nothing to alert"
+  exit 0
+fi
+
+hours=$(( ( $(date +%s) - $(stat -c %Y "$newest") ) / 3600 ))
+if (( hours > MAX_HOURS )); then
+  echo "ALERT: last backup ($newest) is ${hours}h old (max ${MAX_HOURS}h)" >&2
+  exit 1
+fi
+echo "OK: last backup ($newest) is ${hours}h old"