Unauthenticated nested page API leaks restricted & unpublished content

• Location: app/controllers/alchemy/api/pages_controller.rb:28 (Api::PagesController#nested)
• Affected version: Alchemy CMS 8.3.0.dev (Rails 8.1.3)

Description

The unauthenticated GET /api/pages/nested endpoint returns the full page tree to any anonymous caller, including restricted (member-only) pages and unpublished/draft pages that should be hidden.
Appending ?elements=true additionally dumps the element/ingredient content of restricted pages, fully bypassing the access control the sibling show and index actions enforce.

Root cause

Api::PagesController#nested calls no authorize! and applies no published/restricted scoping, unlike show (authorize! :show) and index (accessible_by(current_ability, :index)).
PageTreePreloader loads page.self_and_descendants unfiltered, and PageTreeSerializer emits every page's metadata (and, with elements, public_version.elements) with no ability check.

Evidence

An unauthenticated GET /api/pages/nested returns HTTP 200 with the restricted page ("restricted":true) and an unpublished draft ("public":false); ?elements=true leaks its content (e.g. TOPSECRET_RESTRICTED_BODY_proof123).
The same guest hitting GET /api/pages/3 (show) gets HTTP 403 {"error":"Not authorized"}, proving nested returns what show correctly denies.

Reproduction

# 1) Metadata leak (guest, no auth)
curl -s http://localhost:3000/api/pages/nested | python3 -m json.tool | grep -E '"name"|"restricted"|"public"'

# 2) Content leak of restricted page
curl -s "http://localhost:3000/api/pages/nested?elements=true" | grep -oE 'TOPSECRET_RESTRICTED_BODY_[A-Za-z0-9]+|RESTRICTED_RICHTEXT_[A-Za-z0-9]+'

# 3) Contrast — show denies the same guest
curl -s -o /dev/null -w "show /api/pages/3 -> HTTP %{http_code}\n" http://localhost:3000/api/pages/3

Suggested fix

def nested
  @page = Page.find_by(id: params[:page_id]) || Language.current_root_page
  authorize! :show, @page
  preloaded_page = PageTreePreloader.new(page: @page, user: current_alchemy_user, ability: current_ability).call
  render json: PageTreeSerializer.new(preloaded_page, ability: current_ability,
                                      user: current_alchemy_user, elements: params[:elements])
end

Additionally scope PageTreePreloader's self_and_descendants via accessible_by(current_ability) and gate element emission in PageTreeSerializer#page_elements behind opts[:ability].can?(:show, page).