VSN-2946/feat(hr): carry responsables (managers) across employee/candidate flows

Add the three responsable fields (exploitation / flotte / RH, UUID + cached name) to the employee and candidate DTOs and mappers, and propagate them through every sync path: employee create, candidate→employee→driver on validation, and the bulk from-drivers mapper. Add POST /api/hr/employees/sync-from-driver to apply a driver edit onto the matching employee, and push employee edits back to the driver via /api/user/driver/sync-from-employee (after-commit, best-effort).