Resolve VSN-1986 "Feature/enhance ClassLevel integration with admin MS and migrate from SchoolClass enum"
- Add getClassLevelByCode, getClassLevelCodeByUuid, and getClassLevelUuidByCode via RestTemplate
- Introduce ClassLevelShortResponse model
- Replace SchoolClass enum usage with ClassLevel code/uuid resolution in buildExportDto and startFilteredExport
- Update PassengerDTO, PassengerCircuitSearchDTO, and related mappers to use ClassLevel code/uuid
- Implement ClassLevel code/uuid retrieval in searchPassengers, startFilteredExport, and mapPassengersToDTOs Closes VSN-1986