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/secrets.ex

@@ -1,4 +1,23 @@
 defmodule Mv.Secrets do
+  @moduledoc """
+  Secret provider for AshAuthentication.
+
+  ## Purpose
+  Provides runtime configuration secrets for Ash Authentication strategies,
+  particularly for OIDC (Rauthy) authentication.
+
+  ## Configuration Source
+  Secrets are read from the `:rauthy` key in the application configuration,
+  which is typically set in `config/runtime.exs` from environment variables:
+  - `OIDC_CLIENT_ID`
+  - `OIDC_CLIENT_SECRET`
+  - `OIDC_BASE_URL`
+  - `OIDC_REDIRECT_URI`
+
+  ## Usage
+  This module is automatically called by AshAuthentication when resolving
+  secrets for the User resource's OIDC strategy.
+  """
   use AshAuthentication.Secret
 
   def secret_for(