Refactor Employee-Section relationship

[NikosPoly]

Currently the Employee-Section relationship is inconsistent and bidirectional:

• Employee has a free text String section field instead of a proper reference
• Section stores a List employeeIds creating a bidirectional relationship
  that requires keeping two places in sync

The relationship should be owned by Employee only, same pattern as Loan owning
references to Book and Customer.

Tasks

• Replace String section in Employee with String sectionId referencing the Section MongoDB ID
• Remove employeeIds list from Section — employee owns the relationship
• Add findBySectionId(String sectionId) to EmployeeRepository
• Update EmployeeService and EmployeeController accordingly
• Update SectionService and SectionController accordingly

Note

This is a breaking change — existing employee documents will need migration.