Update to (somewhat?) dendritic pattern

Heavily inspired by https://github.com/Doc-Steve/dendritic-design-with-flake-parts.

> Atomic commits? Never heard of them!

modules/services/borgmatic.nix

@@ -0,0 +1,67 @@
+{
+  flake.modules.homeManager.borgmatic =
+    { config
+    , pkgs
+    , ...
+    }:
+    let
+      hostname = config.networking.hostname;
+      username = config.home.username;
+    in
+    {
+      home.packages = with pkgs; [
+        libnotify
+      ];
+
+      programs.borgmatic = {
+        backups.default = {
+          location = {
+            repositories = [
+              {
+                label = "duke";
+                path = "ssh://borg@duke.veltnet:2222/home/borg/backups/${hostname}-${username}";
+              }
+            ];
+            excludeHomeManagerSymlinks = true;
+            patterns = [
+              #TODO: home.homeDirectory might be usable here
+              "R /home/${username}/Documents"
+              "R /home/${username}/.ssh"
+              "- /home/${username}/Documents/SteamLibrary"
+              "- /home/${username}/Documents/PrismLauncher instances"
+              "- /home/${username}/Documents/.Trash-1000"
+              "- /home/${username}/Documents/VMs/Data"
+              "- **/node_modules"
+              "- **/.git"
+              "- **/.cache"
+              "- **/*build"
+              "- **/target"
+              "- **/.venv"
+              "- **/venv"
+              "- **/.gradle"
+              "- **/out"
+              "- **/dist"
+              "- **/.pio"
+            ];
+          };
+          retention.keepWeekly = 2;
+          storage.extraConfig.ssh_command = "ssh -i /home/${username}/.ssh/id_ed25519_borg";
+          #FIXME: consistency checks!!!
+        };
+      };
+
+      systemd.user.services.borgmatic = {
+        Unit.Description = "Create a backup using borgmatic";
+        Service.ExecStart = "${pkgs.borgmatic}/bin/borgmatic create --stats";
+      };
+
+      systemd.user.timers.borgmatic = {
+        Unit.Description = "Create weekly backups using borgmatic";
+        Timer = {
+          OnCalendar = "Weekly";
+          Persistent = true;
+        };
+        Install.WantedBy = [ "timers.target" ];
+      };
+    };
+}