add btrbk system snapshots

modules/nixos/my_btrbk/default.nix

@@ -0,0 +1,53 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.my_btrbk;
+  inherit (lib) mkEnableOption mkIf mkOption;
+  types = lib.types;
+in
+{
+  options.my_btrbk = {
+    enable = mkEnableOption "Btrbk backups";
+    sshKeyFile = mkOption {
+      type = types.nonEmptyStr;
+    };
+    sshUser = mkOption {
+      type = types.nonEmptyStr;
+      default = "btrbk";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    services.btrbk.instances."remote_falcon" = {
+      onCalendar = "daily";
+      settings = {
+        ssh_identity = cfg.sshKeyFile;
+        ssh_user = cfg.sshUser;
+
+        incremental = "yes";
+        stream_compress = "zstd";
+        stream_compress_level = "3";
+        target_preserve_min = "no";
+        target_preserve = "5d 4w 6m";
+
+        subvolume = {
+          "/" = {
+            snapshot_name = "root";
+          };
+          "/home" = {
+            snapshot_name = "home";
+          };
+          "/nix" = {
+            snapshot_name = "nix";
+          };
+        };
+        snapshot_dir = "/.btrbk_snapshots";
+        target = "send-receive ssh://10.0.0.5/zpool-backup/backups/${config.networking.hostName}";
+      };
+    };
+  };
+}