Changes

tsk69: Implements add/remove favorite story functionality in service
layer
This commit implements the service-layer methods to allow users to add
and remove stories from their favorites. It introduces addFavorite and 
removeFavorite in StoryService, including proper validation for story
existence and user authorization. Integration tests were added to verify
successful addition and removal, as well as error cases for non-existent
stories and unauthorized access. The DBUnit dataset
stories-new-favorited.xml was created to support the tests.