refactor(types): reconcile @specs with their success typings

lib/mv/vereinfacht/client.ex

@@ -8,6 +8,12 @@ defmodule Mv.Vereinfacht.Client do
   """
   require Logger
 
+  @typedoc "Error reasons returned by Vereinfacht API calls."
+  @type error_reason ::
+          :not_configured
+          | {:request_failed, map()}
+          | {:http, non_neg_integer(), :html_response | binary()}
+
   @content_type "application/vnd.api+json"
 
   @doc """
@@ -31,7 +37,7 @@ defmodule Mv.Vereinfacht.Client do
       {:error, :not_configured}
   """
   @spec test_connection(String.t() | nil, String.t() | nil, String.t() | nil) ::
-          {:ok, :connected} | {:error, term()}
+          {:ok, :connected} | {:error, error_reason()}
   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}
@@ -230,7 +236,7 @@ defmodule Mv.Vereinfacht.Client do
 
   Returns the full response body (decoded JSON) for debugging/display.
   """
-  @spec get_contact(String.t()) :: {:ok, map()} | {:error, term()}
+  @spec get_contact(String.t()) :: {:ok, map()} | {:error, error_reason()}
   def get_contact(contact_id) when is_binary(contact_id) do
     fetch_contact(contact_id, [])
   end

lib/mv/vereinfacht/vereinfacht.ex

@@ -26,7 +26,7 @@ defmodule Mv.Vereinfacht do
   - `{: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()}
+  @spec test_connection() :: {:ok, :connected} | {:error, Mv.Vereinfacht.Client.error_reason()}
   def test_connection do
     Client.test_connection(
       Mv.Config.vereinfacht_api_url(),