Vereinfacht API: use filter for contact lookup, drop extra required fields

- find_contact_by_email uses GET with filter[isExternal]=true and filter[email]
- vereinfacht_required_member_fields is now empty (API accepts minimal payload)

test/mv/vereinfacht/client_test.exs

@@ -30,6 +30,13 @@ defmodule Mv.Vereinfacht.ClientTest do
     end
   end
 
+  describe "find_contact_by_email/1" do
+    test "returns {:error, :not_configured} when Vereinfacht is not configured" do
+      assert Client.find_contact_by_email("kayley.becker@example.com") ==
+               {:error, :not_configured}
+    end
+  end
+
   defp build_member_struct do
     %{
       first_name: "Test",