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/membership/membership.ex

@@ -1,4 +1,21 @@
 defmodule Mv.Membership do
+  @moduledoc """
+  Ash Domain for membership management.
+
+  ## Resources
+  - `Member` - Club members with personal information and custom properties
+  - `Property` - Dynamic custom field values attached to members
+  - `PropertyType` - Schema definitions for custom properties
+
+  ## Public API
+  The domain exposes these main actions:
+  - Member CRUD: `create_member/1`, `list_members/0`, `update_member/2`, `destroy_member/1`
+  - Property management: `create_property/1`, `list_property/0`, etc.
+  - PropertyType management: `create_property_type/1`, `list_property_types/0`, etc.
+
+  ## Admin Interface
+  The domain is configured with AshAdmin for management UI.
+  """
   use Ash.Domain,
     extensions: [AshAdmin.Domain, AshPhoenix]