fix: add a robots.txt endpoint

lumi2/static/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /

lumi2/usermanager.py

@@ -92,6 +92,13 @@ def about():
     return render_template('usermanager/about.html')
 
 
+@bp.route('/robots.txt')
+def robots():
+    """Serves the 'robots.txt' file."""
+
+    return send_from_directory(current_app.static_folder, "robots.txt")
+
+
 @bp.route("/users/view/<string:username>")
 @login_required
 def user_view(username: str):