nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/services/postgresql-backup/default.nix

@@ -0,0 +1,28 @@
+# Backup your data, kids!
+{ config, lib, ... }:
+let
+  cfg = config.my.services.postgresql-backup;
+in
+{
+  options.my.services.postgresql-backup = {
+    enable = lib.mkEnableOption "Backup SQL databases";
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.postgresqlBackup = {
+      enable = true;
+      backupAll = true;
+      location = "/var/backup/postgresql";
+    };
+
+    my.services.backup = {
+      paths = [
+        config.services.postgresqlBackup.location
+      ];
+      # No need to store previous backups thanks to `restic`
+      exclude = [
+        (config.services.postgresqlBackup.location + "/*.prev.sql.gz")
+      ];
+    };
+  };
+}