Fix backend user contract and add User-Employee-Customer linkage

- 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

petshop-api.postman_collection.json

@@ -1435,7 +1435,7 @@
             ],
             "body": {
               "mode": "raw",
-              "raw": "{\n  \"username\": \"newuser\",\n  \"password\": \"password123\",\n  \"role\": \"STAFF\"\n}"
+              "raw": "{\n  \"username\": \"newuser\",\n  \"password\": \"password123\",\n  \"fullName\": \"New User\",\n  \"email\": \"newuser@petshop.com\",\n  \"role\": \"STAFF\",\n  \"active\": true\n}"
             }
           }
         },
@@ -1457,7 +1457,7 @@
             ],
             "body": {
               "mode": "raw",
-              "raw": "{\n  \"username\": \"user1\",\n  \"role\": \"STAFF\"\n}"
+              "raw": "{\n  \"username\": \"user1\",\n  \"password\": \"newpassword123\",\n  \"fullName\": \"Updated User\",\n  \"email\": \"user1@petshop.com\",\n  \"role\": \"STAFF\",\n  \"active\": true\n}"
             }
           }
         },