Change /api/v1/timelines/public to require auth when public preview is off (#11802)

Fixes #11289

app/controllers/api/v1/timelines/public_controller.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 class Api::V1::Timelines::PublicController < Api::BaseController
+  before_action :require_user!, only: [:show], if: :require_auth?
   after_action :insert_pagination_headers, unless: -> { @statuses.empty? }
 
   respond_to :json
@@ -12,6 +13,10 @@ class Api::V1::Timelines::PublicController < Api::BaseController
 
   private
 
+  def require_auth?
+    !Setting.timeline_preview
+  end
+
   def load_statuses
     cached_public_statuses
   end