nixos/restic-rest-server: Autocreate empty .htpasswd if needed for service boot

When `privateRepos = true`, the service will not start if the `.htpasswd` does not exist.
Use `systemd-tmpfiles` to autocreate an (empty) file to ensure the service can boot
before actual `htpasswd` contents are registered.

This is safe as restic-rest-server will deny all entry if the file is empty.

nixos/modules/services/backup/restic-rest-server.nix

@@ -95,6 +95,10 @@ in
       };
     };
 
+    systemd.tmpfiles.rules = mkIf cfg.privateRepos [
+        "f ${cfg.dataDir}/.htpasswd 0700 restic restic -"
+    ];
+
     users.users.restic = {
       group = "restic";
       home = cfg.dataDir;