backup: migrate to native services.restic.backups

Replace custom backup-helper flake input with NixOS native
services.restic.backups module for ha1, monitoring01, and nixos-test1.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hosts/nixos-test1/configuration.nix

@@ -51,15 +51,25 @@
   networking.firewall.enable = false;
 
   # Secrets
-  # Backup helper
+  # Backup
   sops.secrets."backup_helper_secret" = { };
-  backup-helper = {
-    enable = true;
-    password-file = "/run/secrets/backup_helper_secret";
-    backup-dirs = [
+  services.restic.backups.test = {
+    repository = "rest:http://10.69.12.52:8000/backup-nix";
+    passwordFile = "/run/secrets/backup_helper_secret";
+    paths = [
       "/etc/machine-id"
       "/etc/os-release"
     ];
+    timerConfig = {
+      OnCalendar = "daily";
+      Persistent = true;
+    };
+    pruneOpts = [
+      "--keep-daily 7"
+      "--keep-weekly 4"
+      "--keep-monthly 6"
+      "--keep-within 1d"
+    ];
   };
 
   system.stateVersion = "23.11"; # Did you read the comment?