docs: enable Credo ModuleDoc check and fix remaining modules

Add @moduledoc to Secrets, LiveHelpers, AuthOverrides, and Membership domain.
Enable Credo.Check.Readability.ModuleDoc in .credo.exs.

lib/mv_web/live_helpers.ex

@@ -1,4 +1,16 @@
 defmodule MvWeb.LiveHelpers do
+  @moduledoc """
+  Shared LiveView lifecycle hooks and helper functions.
+
+  ## on_mount Hooks
+  - `:default` - Sets the user's locale from session (defaults to "de")
+
+  ## Usage
+  Add to LiveView modules via:
+  ```elixir
+  on_mount {MvWeb.LiveHelpers, :default}
+  ```
+  """
   def on_mount(:default, _params, session, socket) do
     locale = session["locale"] || "de"
     Gettext.put_locale(locale)