docs: add @moduledoc to UserAuth and UserNotifier

Resolves credo readability warnings for missing module documentation.

app/lib/firehose_web/user_auth.ex

@@ -1,4 +1,12 @@
 defmodule FirehoseWeb.UserAuth do
+  @moduledoc """
+  Handles user authentication for the web layer.
+
+  Provides plugs and callbacks for logging users in and out,
+  managing session tokens, enforcing authentication requirements,
+  and integrating with LiveView via `on_mount` callbacks.
+  """
+
   use FirehoseWeb, :verified_routes
 
   import Plug.Conn