Implement REST API for user management with admin authentication

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This PR implements a complete Java REST API for user management based on the requirements specification. The API allows administrators to retrieve user information by ID with proper authentication and error handling.

Key Features Implemented

REST API Endpoint

• GET /api/users/{id} - Retrieves user information by ID
• Returns user details including name, email, phone, and department
• Proper HTTP status codes (200, 404, 401, 403)

Security & Authentication

• Spring Security with Basic Authentication
• Admin role required for API access
• Default admin credentials: admin / admin123
• Unauthorized requests return 401, insufficient privileges return 403

Data Layer

• JPA User entity with fields: id, name, email, phone, department
• H2 in-memory database for development
• Pre-loaded with 5 sample Japanese users
• Proper error handling for non-existent users (404 response)

Testing

• Comprehensive unit tests covering all scenarios:
  • Successful user retrieval with admin credentials
  • 404 error for non-existent users
  • 401 error for unauthenticated requests
  • 403 error for non-admin users

Usage Example

# Get user information (successful case)
curl -u admin:admin123 http://localhost:8080/api/users/1

# Response:
{
  "id": 1,
  "name": "田中太郎",
  "email": "tanaka@example.com",
  "phone": "090-1234-5678",
  "department": "開発部"
}

# Non-existent user (404 error)
curl -u admin:admin123 http://localhost:8080/api/users/999

# Unauthenticated request (401 error)
curl http://localhost:8080/api/users/1

Technical Stack

• Java 17 with Spring Boot 3.1.5
• Spring Security for authentication
• Spring Data JPA for data access
• H2 database for development
• Maven for build management
• Comprehensive test suite

The implementation follows Spring Boot best practices with proper separation of concerns across controller, service, and repository layers. All requirements from the specification have been fulfilled with additional documentation and testing.

Fixes #1.

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