The stale postmaster.pid file from the backup is blocking Postgres from starting. Since you can't shell into a crashed service, the easiest fix is to deploy a simple Alpine container, mount the same volume to it, run rm /var/lib/postgresql/data/postmaster.pid, then detach the volume and reattach it to your Postgres service. Alternatively, you could spin up a fresh Postgres instance and restore your backup there instead—that sidesteps the pid file issue entirely.

You still need to unmount the volume from the Postgres service and mount it to the Filebrowser template.
There's also no 'Volumes' tab in a service.

Please be careful with what you recommend to other users.