- Add active field to User entity and users table
- Add userId linkage to Employee and Customer entities with unique constraints and FKs
- Add repository methods findByUserId and findAllByEmail
- Create UserBusinessLinkageService for shared employee/customer creation logic
- Create AuthenticationHelper utility for resolving authenticated users
- Update UserService to persist all user fields and create linked business entities
- Update AuthController register to set active and create linked customer
- Update DataInitializer to be idempotent and use shared linkage service
- Update Postman collection user endpoints with fullName, email, and active
