feat: add Vereinfacht connection test button to settings

lib/mv/vereinfacht/vereinfacht.ex

@@ -14,6 +14,27 @@ defmodule Mv.Vereinfacht do
   alias Mv.Helpers.SystemActor
   alias Mv.Helpers
 
+  @doc """
+  Tests the connection to the Vereinfacht API using the current configuration.
+
+  Delegates to `Mv.Vereinfacht.Client.test_connection/3` with the values from
+  `Mv.Config` (ENV variables take priority over database settings).
+
+  ## Returns
+  - `{:ok, :connected}` – credentials are valid and API is reachable
+  - `{:error, :not_configured}` – URL, API key or club ID is missing
+  - `{:error, {:http, status, message}}` – API returned an error (e.g. 401, 403)
+  - `{:error, {:request_failed, reason}}` – network/transport error
+  """
+  @spec test_connection() :: {:ok, :connected} | {:error, term()}
+  def test_connection do
+    Client.test_connection(
+      Mv.Config.vereinfacht_api_url(),
+      Mv.Config.vereinfacht_api_key(),
+      Mv.Config.vereinfacht_club_id()
+    )
+  end
+
   @doc """
   Syncs a single member to Vereinfacht (create or update finance contact).