Make Nextcloud NAS backups independent, verified and retryable

systemd/user/nextcloud-borg-backup.timer

@@ -1,10 +1,12 @@
 [Unit]
-Description=Daily Nextcloud Borg backup timer
+Description=Check every 15 minutes for due or failed Nextcloud NAS backup
 
 [Timer]
-OnCalendar=*-*-* 03:15:00
+OnStartupSec=2min
+OnCalendar=*:0/15
 Persistent=true
-RandomizedDelaySec=30m
+RandomizedDelaySec=60s
+AccuracySec=30s
 Unit=nextcloud-borg-backup.service
 
 [Install]