User or Device 'Last Seen' /'Checkin'

[SCUZNUTS]

It would be good to know if my partner or family member phones etc are still checking in and uploading on the regular.

[jtagcat]

adding search dedupe keywords: backups backing up regularly admin battery optimization backup status

I'm planning to replace Syncthing with Immich for the family device fleet. Feature parity would be:

• Showing count/date of unsynced items (per device, not user!)
  
• Showing last seen (already exists)
  
• Exposing the overview in a single place (not collecting passwords and logging in to each one). Could be a dashboard, could be Prometheus.

Some devices check in once every week or two, some do it every day. I need to know if there is intervention required, and for better over-the-phone instructions, what caused it (is the app dead and just needs to be opened, is WiFi always off while charging (some always stay on mobile data).