Changes

tsk59: Implements story creation and update in service layer
This commit adds full support for creating and updating stories. It
introduces
`StoryCreateDto`, implements creation and update flows in
`StoryService`, and applies permission checks using the security role
caller for CREATOR actions. A new `UserService` is added to resolve the
current user. The `Story` entity now supports relaxed nullability for
drafts, immutability after publication, theme and text validations, and
a new `isComplete` method. The database schema is updated to match these
rules. The `StoryMapper` is adjusted for the updated entity constructor.
Comprehensive unit and integration tests verify creation, updating,
validation behavior, and authorization handling.