refactor: unify referenceProjectUuid and add Jira imported projects endpoints
- Replace projectId with referenceProjectUuid across all DTOs, services, repositories, and controllers
- Add POST /jira/imported-projects and GET /jira/imported-projects endpoints
- Fix missing @Transactional annotations and exception mappings
- Update DTO validations and clean obsolete code
- Rename importedProjectUuid and alreadyImported for clearer semantics