Android: app stops receiving new messages in the background, requires restart

## Summary

On Android, the app stops receiving new messages while it is in the background. Messages only appear after a full restart of the app.

## Steps to reproduce

1. Open the Buzz Android app and view a channel.
2. Background the app (or leave the screen off) for a while.
3. Have someone post new messages to that channel meanwhile.
4. Return to the app.

## Expected

New messages have arrived, or arrive shortly after returning to the app.

## Actual

The channel still shows the old state. New messages do not appear until the app is force-closed and reopened. Pull-to-refresh does not reliably bring them in either.

## Notes

It looks like the relay subscription is dropped when the app is backgrounded and is only re-established on a cold start, rather than reconnecting on resume.

Battery optimisation was checked on the device and is not the cause.

Happy to provide device model, Android version and app build number if that helps narrow it down.