Build an Employee Directory API

Problem/Feature Description

An HR department needs a REST API to manage their employee directory. Build it using Spring Boot and Java.

Resources:

• Department: { id, name, managerId }
• Employee: { id, firstName, lastName, email, departmentId, jobTitle, salary, hireDate }

Endpoints:

• GET /api/departments -- list all departments
• POST /api/departments -- create a department (name required, must be unique)
• GET /api/departments/{id} -- get department details including employee count
• GET /api/departments/{id}/employees -- list employees in a department, support pagination (page, size params)
• POST /api/employees -- create an employee (firstName, lastName, email required; email must be unique; departmentId must reference an existing department; salary must be positive)
• GET /api/employees/{id} -- get an employee by ID
• PUT /api/employees/{id} -- update employee details
• DELETE /api/employees/{id} -- remove an employee (cannot delete a department manager)
• PATCH /api/employees/{id}/transfer -- transfer an employee to a different department (body: { departmentId })

Business rules:

• Department manager must be an employee within that department
• An employee's email must be unique across the entire organization
• Salary must be between 30000 and 500000
• Cannot delete a department that has employees

Use H2 embedded database with Spring Data JPA.

Output Specification

Produce a complete Spring Boot project:

• src/main/java/com/example/ -- controller, service, model/entity, and repository packages
• pom.xml or build.gradle -- with dependencies
• src/main/resources/application.properties -- configuration including H2 settings

You may create additional files as needed for a well-structured codebase.