html: add noindex tag to header.html

should stop instances being indexed by search engines.

html/header.html

@@ -2,6 +2,7 @@
 <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
 <meta name="Description" content="jfa-go, a better way to manage Jellyfin users.">
 <meta name="color-scheme" content="dark light">
+<meta name="robots" content="noindex">
 
 <link rel="apple-touch-icon" sizes="180x180" href="{{ .urlBase }}/apple-touch-icon.png">
 <link rel="icon" type="image/png" sizes="32x32" href="{{ .urlBase }}/favicon-32x32.png">