Adds LNbits Borg backup module

Implements a new module for backing up LNbits data using Borg.

This module automates hourly backups, encrypts the data, and provides point-in-time recovery. It includes scripts for listing, restoring, and mounting backups. A comprehensive setup guide is provided in the documentation.

The configuration allows specifying the Borg repository location, schedule, compression settings, retention policy, and SSH key for secure access.

config/machines/example-machine/configuration.nix

@@ -16,6 +16,10 @@ in
     # Import boot configuration (bootloader settings)
     ./boot.nix
 
+    # BORG backup
+    # read docs/lnbits-borg-backup-guide.md
+    # ./borg-lnbits.nix
+
     # Import any machine-specific services
     # Comment out or remove if not needed
     # ./example-service.nix