feat: add Vereinfacht connection test button to settings

lib/mv/vereinfacht/client.ex

@@ -10,6 +10,54 @@ defmodule Mv.Vereinfacht.Client do
 
   @content_type "application/vnd.api+json"
 
+  @doc """
+  Tests the connection to the Vereinfacht API with the given credentials.
+
+  Makes a lightweight `GET /finance-contacts?page[size]=1` request to verify
+  that the API URL, API key, and club ID are valid and reachable.
+
+  ## Returns
+  - `{:ok, :connected}` – credentials are valid (HTTP 200)
+  - `{:error, :not_configured}` – any parameter is nil or blank
+  - `{:error, {:http, status, message}}` – API returned an error (e.g. 401, 403)
+  - `{:error, {:request_failed, reason}}` – network/transport error
+
+  ## Examples
+
+      iex> test_connection("https://api.example.com/api/v1", "token", "2")
+      {:ok, :connected}
+
+      iex> test_connection(nil, "token", "2")
+      {:error, :not_configured}
+  """
+  @spec test_connection(String.t() | nil, String.t() | nil, String.t() | nil) ::
+          {:ok, :connected} | {:error, term()}
+  def test_connection(api_url, api_key, club_id) do
+    if blank?(api_url) or blank?(api_key) or blank?(club_id) do
+      {:error, :not_configured}
+    else
+      url =
+        api_url
+        |> String.trim_trailing("/")
+        |> then(&"#{&1}/finance-contacts?page[size]=1")
+
+      case Req.get(url, [headers: headers(api_key)] ++ req_http_options()) do
+        {:ok, %{status: 200}} ->
+          {:ok, :connected}
+
+        {:ok, %{status: status, body: body}} ->
+          {:error, {:http, status, extract_error_message(body)}}
+
+        {:error, reason} ->
+          {:error, {:request_failed, reason}}
+      end
+    end
+  end
+
+  defp blank?(nil), do: true
+  defp blank?(s) when is_binary(s), do: String.trim(s) == ""
+  defp blank?(_), do: true
+
   @doc """
   Creates a finance contact in Vereinfacht for the given member.
 
@@ -360,5 +408,16 @@ defmodule Mv.Vereinfacht.Client do
   defp extract_error_message(%{"errors" => [%{"detail" => d} | _]}) when is_binary(d), do: d
   defp extract_error_message(%{"errors" => [%{"title" => t} | _]}) when is_binary(t), do: t
   defp extract_error_message(body) when is_map(body), do: inspect(body)
+
+  defp extract_error_message(body) when is_binary(body) do
+    trimmed = String.trim(body)
+
+    if String.starts_with?(trimmed, "<") do
+      :html_response
+    else
+      trimmed
+    end
+  end
+
   defp extract_error_message(other), do: inspect(other)
 end

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).