Add mapper layer for DTO to model conversion

[NikosPoly]

Currently DTO to model conversion is done manually inside the service layer,
mixing conversion logic with business logic.

A dedicated mapper layer would improve separation of concerns and make the
code cleaner and easier to maintain.

Tasks

• Create mapper classes for each resource:
  • BookMapper
  • CustomerMapper
  • EmployeeMapper
  • LoanMapper
  • SectionMapper
• Move all DTO to model and model to DTO conversion logic out of services into mappers
• Consider using MapStruct library to auto-generate mapper implementations