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)

lib/mv/vereinfacht/client.ex

@@ -166,12 +166,12 @@ defmodule Mv.Vereinfacht.Client do
   end
 
   @doc """
-  Finds a finance contact by email (GET /finance-contacts, then match in response).
+  Finds a finance contact by email using the API filter.
 
-  The Vereinfacht API does not allow filter by email on this endpoint, so we
-  fetch the first page and find the contact client-side. Returns {:ok, contact_id}
-  if a contact with that email exists, {:error, :not_found} if none, or
-  {:error, reason} on API/network failure. Used before create for idempotency.
+  Uses GET /finance-contacts?filter[isExternal]=true&filter[email]=... so the API
+  returns only matching external contacts. Returns {:ok, contact_id} if a contact
+  exists, {:error, :not_found} if none, or {:error, reason} on API/network failure.
+  Used before create for idempotency.
   """
   @spec find_contact_by_email(String.t()) :: {:ok, String.t()} | {:error, :not_found | term()}
   def find_contact_by_email(email) when is_binary(email) do
@@ -182,25 +182,17 @@ defmodule Mv.Vereinfacht.Client do
     end
   end
 
-  @find_contact_page_size 100
-  @find_contact_max_pages 100
-
   defp do_find_contact_by_email(email) do
-    normalized = String.trim(email) |> String.downcase()
-    do_find_contact_by_email_page(1, normalized)
-  end
-
-  defp do_find_contact_by_email_page(page, _normalized) when page > @find_contact_max_pages do
-    {:error, :not_found}
-  end
-
-  defp do_find_contact_by_email_page(page, normalized) do
     base = base_url() |> String.trim_trailing("/") |> then(&"#{&1}/finance-contacts")
-    url = base <> "?page[size]=#{@find_contact_page_size}&page[number]=#{page}"
+    encoded_email = URI.encode_www_form(email |> String.trim())
+    url = "#{base}?filter[isExternal]=true&filter[email]=#{encoded_email}"
 
     case Req.get(url, [headers: headers(api_key())] ++ req_http_options()) do
       {:ok, %{status: 200, body: body}} when is_map(body) ->
-        handle_find_contact_page_response(body, page, normalized)
+        case get_first_contact_id_from_list(body) do
+          nil -> {:error, :not_found}
+          id -> {:ok, id}
+        end
 
       {:ok, %{status: status, body: body}} ->
         {:error, {:http, status, extract_error_message(body)}}
@@ -210,48 +202,12 @@ defmodule Mv.Vereinfacht.Client do
     end
   end
 
-  defp handle_find_contact_page_response(body, page, normalized) do
-    case find_contact_id_by_email_in_list(body, normalized) do
-      id when is_binary(id) -> {:ok, id}
-      nil -> maybe_find_contact_next_page(body, page, normalized)
-    end
+  defp get_first_contact_id_from_list(%{"data" => [%{"id" => id} | _]}) do
+    normalize_contact_id(id)
   end
 
-  defp maybe_find_contact_next_page(body, page, normalized) do
-    data = Map.get(body, "data") || []
-
-    if length(data) < @find_contact_page_size,
-      do: {:error, :not_found},
-      else: do_find_contact_by_email_page(page + 1, normalized)
-  end
-
-  defp find_contact_id_by_email_in_list(%{"data" => list}, normalized) when is_list(list) do
-    Enum.find_value(list, fn
-      %{"id" => id, "attributes" => %{"email" => att_email, "isExternal" => true}}
-      when is_binary(att_email) ->
-        if att_email |> String.trim() |> String.downcase() == normalized do
-          normalize_contact_id(id)
-        else
-          nil
-        end
-
-      %{"id" => id, "attributes" => %{"email" => att_email, "isExternal" => "true"}}
-      when is_binary(att_email) ->
-        if att_email |> String.trim() |> String.downcase() == normalized do
-          normalize_contact_id(id)
-        else
-          nil
-        end
-
-      %{"id" => _id, "attributes" => _} ->
-        nil
-
-      _ ->
-        nil
-    end)
-  end
-
-  defp find_contact_id_by_email_in_list(_, _), do: nil
+  defp get_first_contact_id_from_list(%{"data" => []}), do: nil
+  defp get_first_contact_id_from_list(_), do: nil
 
   defp normalize_contact_id(id) when is_binary(id), do: id
   defp normalize_contact_id(id) when is_integer(id), do: to_string(id)