Build an Employee Directory API

Problem/Feature Description

An HR department needs a simple employee directory backend. Build it in Go using the standard library.

Resources:

• Department: { id, name, managerId }
• Employee: { id, name, email, departmentId, role, hireDate }

Endpoints:

• GET /api/departments -- list all departments
• POST /api/departments -- create a department (name required, must be unique)
• GET /api/departments/{id}/employees -- list employees in a department
• GET /api/employees -- list all employees, with optional ?department={id} filter
• GET /api/employees/{id} -- get a single employee
• POST /api/employees -- create an employee (name, email required; email must be unique; departmentId must reference an existing department)
• PUT /api/employees/{id} -- update an employee
• DELETE /api/employees/{id} -- remove an employee (if the employee is a department manager, reject the deletion)

Seed with 2 departments and 5 employees. Use in-memory storage.

Output Specification

Produce:

• main.go -- entry point
• Source files for handlers, models, errors, and middleware
• go.mod

Organize files as appropriate.