Add locking, backup pruning, and systemd timer for release checks

systemd/pikit-update.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Pi-Kit release update check/apply helper
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/curl -s -H "Content-Type: application/json" -X POST http://127.0.0.1/api/update/check
+User=root
+Group=root
+TimeoutStartSec=300
+
+[Install]
+WantedBy=multi-user.target

systemd/pikit-update.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Daily Pi-Kit release update check
+
+[Timer]
+OnCalendar=*-*-* 04:20:00
+RandomizedDelaySec=900
+Persistent=true
+Unit=pikit-update.service
+
+[Install]
+WantedBy=timers.target