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