Customer and employee ID format validation and uniqueness

[NikosPoly]

Currently customerId and employeeId accept any string value with no format enforcement
or uniqueness guarantee. Gender also accepts any string value.

Tasks

• Convert gender field in Customer to a Gender enum (MALE, FEMALE, OTHER)
• Add @pattern validation on customerId — format CUST\d{4} e.g. CUST0001
• Add @pattern validation on employeeId — format EMP\d{4} e.g. EMP0001
• Add @indexed(unique = true) on:
  • customerId and email in Customer
  • employeeId in Employee
  • isbn in Book