feat: add join form

lib/mv_web/plugs/join_form_enabled.ex

@@ -0,0 +1,33 @@
+defmodule MvWeb.Plugs.JoinFormEnabled do
+  @moduledoc """
+  For GET /join: returns 404 when the join form is disabled in settings.
+  No-op for other paths.
+  """
+  import Plug.Conn
+
+  alias Mv.Membership
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    if join_path?(conn), do: maybe_404(conn), else: conn
+  end
+
+  defp join_path?(conn) do
+    conn.request_path == "/join" and conn.method == "GET"
+  end
+
+  defp maybe_404(conn) do
+    case Membership.get_settings() do
+      {:ok, %{join_form_enabled: true}} -> conn
+      _ -> send_404(conn)
+    end
+  end
+
+  defp send_404(conn) do
+    conn
+    |> put_resp_content_type("text/html")
+    |> send_resp(404, "Not Found")
+    |> halt()
+  end
+end