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/auth_overrides.ex

@@ -1,4 +1,16 @@
 defmodule MvWeb.AuthOverrides do
+  @moduledoc """
+  UI customizations for AshAuthentication Phoenix components.
+
+  ## Overrides
+  - `SignIn` - Restricts form width to prevent full-width display
+  - `Banner` - Replaces default logo with "Mitgliederverwaltung" text
+  - `HorizontalRule` - Translates "or" text to German
+
+  ## Documentation
+  For complete reference on available overrides, see:
+  https://hexdocs.pm/ash_authentication_phoenix/ui-overrides.html
+  """
   use AshAuthentication.Phoenix.Overrides
   use Gettext, backend: MvWeb.Gettext
 

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)