refactor: modularize backup tools

Extract backup tools from core user packages into optional module:

- Add backup solutions: borgbackup, borgmatic, rclone

Author: refnode

users/refnode/common/optional/backup.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+
+{
+  # Backup and synchronization tools
+  
+  home.packages = with pkgs; [
+    # Backup solutions
+    borgbackup
+    borgmatic
+    rclone
+  ];
+}

users/refnode/default.nix

@@ -5,6 +5,7 @@
     ./common/optional/kubernetes.nix
     ./common/optional/rust.nix
     ./common/optional/mailtools.nix
+    ./common/optional/backup.nix
   ];
 
   # specify my home-manager configs
@@ -46,10 +47,6 @@
     # crypto
     gnupg
     pwgen
-    # backup tools
-    borgbackup
-    borgmatic
-    rclone
     tcptraceroute
     unixtools.watch
     fswatch