feat(vereinfacht): add DB schema, config and setting attributes

- Migrations: vereinfacht_contact_id on members, vereinfacht_* on settings
- Mv.Config: Vereinfacht ENV/Settings helpers, vereinfacht_configured?, contact_view_url
- Setting: vereinfacht_api_url, api_key, club_id

lib/mv/config.ex

@@ -142,4 +142,98 @@ defmodule Mv.Config do
     |> Keyword.get(key, default)
     |> parse_and_validate_integer(default)
   end
+
+  # ---------------------------------------------------------------------------
+  # Vereinfacht accounting software integration
+  # ENV variables take priority; fallback to Settings from database.
+  # ---------------------------------------------------------------------------
+
+  @doc """
+  Returns the Vereinfacht API base URL.
+
+  Reads from `VEREINFACHT_API_URL` env first, then from Settings.
+  """
+  @spec vereinfacht_api_url() :: String.t() | nil
+  def vereinfacht_api_url do
+    env_or_setting("VEREINFACHT_API_URL", :vereinfacht_api_url)
+  end
+
+  @doc """
+  Returns the Vereinfacht API key (Bearer token).
+
+  Reads from `VEREINFACHT_API_KEY` env first, then from Settings.
+  """
+  @spec vereinfacht_api_key() :: String.t() | nil
+  def vereinfacht_api_key do
+    env_or_setting("VEREINFACHT_API_KEY", :vereinfacht_api_key)
+  end
+
+  @doc """
+  Returns the Vereinfacht club ID for multi-tenancy.
+
+  Reads from `VEREINFACHT_CLUB_ID` env first, then from Settings.
+  """
+  @spec vereinfacht_club_id() :: String.t() | nil
+  def vereinfacht_club_id do
+    env_or_setting("VEREINFACHT_CLUB_ID", :vereinfacht_club_id)
+  end
+
+  @doc """
+  Returns true if Vereinfacht is fully configured (URL, API key, and club ID all set).
+  """
+  @spec vereinfacht_configured?() :: boolean()
+  def vereinfacht_configured? do
+    present?(vereinfacht_api_url()) and present?(vereinfacht_api_key()) and
+      present?(vereinfacht_club_id())
+  end
+
+  @doc """
+  Returns true if any Vereinfacht ENV variable is set (used to gray out Settings UI).
+  """
+  @spec vereinfacht_env_configured?() :: boolean()
+  def vereinfacht_env_configured? do
+    System.get_env("VEREINFACHT_API_URL") != nil or
+      System.get_env("VEREINFACHT_API_KEY") != nil or
+      System.get_env("VEREINFACHT_CLUB_ID") != nil
+  end
+
+  defp env_or_setting(env_key, setting_key) do
+    case System.get_env(env_key) do
+      nil -> get_vereinfacht_from_settings(setting_key)
+      value -> trim_nil(value)
+    end
+  end
+
+  defp get_vereinfacht_from_settings(key) do
+    case Mv.Membership.get_settings() do
+      {:ok, settings} -> settings |> Map.get(key) |> trim_nil()
+      {:error, _} -> nil
+    end
+  end
+
+  defp trim_nil(nil), do: nil
+
+  defp trim_nil(s) when is_binary(s) do
+    t = String.trim(s)
+    if t == "", do: nil, else: t
+  end
+
+  @doc """
+  Returns the URL to view a finance contact (e.g. in Vereinfacht frontend or API).
+
+  Uses the configured API base URL and appends /finance-contacts/{id}.
+  Can be extended later with a dedicated frontend URL setting.
+  """
+  @spec vereinfacht_contact_view_url(String.t()) :: String.t() | nil
+  def vereinfacht_contact_view_url(contact_id) when is_binary(contact_id) do
+    base = vereinfacht_api_url()
+
+    if present?(base),
+      do: base |> String.trim_trailing("/") |> then(&"#{&1}/finance-contacts/#{contact_id}"),
+      else: nil
+  end
+
+  defp present?(nil), do: false
+  defp present?(s) when is_binary(s), do: String.trim(s) != ""
+  defp present?(_), do: false
 end