snapper: Add persistentTimer option to config

Defaults to false, but allows users to enable it for machines that
aren't on persistently (e.g., laptops, home PCs).

nixos/modules/services/misc/snapper.nix

@@ -103,6 +103,18 @@ in
       '';
     };
 
+    persistentTimer = mkOption {
+      default = false;
+      type = types.bool;
+      example = true;
+      description = ''
+        Set the `persistentTimer` option for the
+        {manpage}`systemd.timer(5)`
+        which triggers the snapshot immediately if the last trigger
+        was missed (e.g. if the system was powered down).
+      '';
+    };
+
     cleanupInterval = mkOption {
       type = types.str;
       default = "1d";
@@ -198,7 +210,14 @@ in
       inherit documentation;
       requires = [ "local-fs.target" ];
       serviceConfig.ExecStart = "${pkgs.snapper}/lib/snapper/systemd-helper --timeline";
-      startAt = cfg.snapshotInterval;
+    };
+
+    systemd.timers.snapper-timeline = {
+      wantedBy = [ "timers.target" ];
+      timerConfig = {
+        Persistent = cfg.persistentTimer;
+        OnCalendar = cfg.snapshotInterval;
+      };
     };
 
     systemd.services.snapper-cleanup = {