Add the redirect route.

lib/link_shortener_web/controllers/redirect_controller.ex

@@ -0,0 +1,9 @@
+defmodule LinkShortenerWeb.RedirectController do
+  use LinkShortenerWeb, :controller
+  alias LinkShortener.Links
+
+  def show(conn, %{"shorten" => shorten}) do
+    %{url: url} = Links.get_one_by_shorten(shorten)
+    redirect(conn, external: url)
+  end
+end