feat(modules/nixos/base,hosts): add btrbk for filesystem backup

modules/nixos/base/btrbk.nix

@@ -0,0 +1,24 @@
+{
+  # Tool for creating snapshots and remote backups of btrfs subvolumes
+  # https://github.com/digint/btrbk
+  services.btrbk.instances.btrbk = {
+    # How often this btrbk instance is started. See systemd.time(7) for more information about the format.
+    onCalendar = "daily";
+    settings = {
+      # keep daily snapshots for 14 days
+      snapshot_preserve = "14d";
+      # keep all snapshots for 2 days, no matter how frequently you (or your cron job) run btrbk
+      snapshot_preserve_min = "2d";
+      volume = {
+        "/btr_pool" = {
+          subvolume = {
+            "@persistent" = {
+              snapshot_create = "always";
+            };
+          };
+          target = "/snapshots";
+        };
+      };
+    };
+  };
+}