Implement REST API for user management with JWT authentication

[Copilot]

This PR implements a complete REST API for user management based on the detailed specification provided in the issue. The implementation includes a Spring Boot application with JWT-based authentication, comprehensive error handling, and full test coverage.

Key Features Implemented

API Endpoint: GET /api/users/{id}

• Retrieves user information by ID
• Requires JWT Bearer token authentication
• Validates path parameters (ID must be Long >= 0)
• Returns user data in JSON format with id, name, email, and role fields

Authentication & Authorization:

• JWT-based security implementation using Spring Security
• Custom JWT authentication filter
• Proper token validation and user role extraction
• Helper endpoint /auth/token for generating test tokens

Error Handling:

• 400 Bad Request: Invalid ID format or negative values
• 401 Unauthorized: Missing or invalid JWT token
• 403 Forbidden: Insufficient permissions (handled by security config)
• 404 Not Found: User does not exist

Database Integration:

• H2 in-memory database for demonstration
• JPA/Hibernate entity mapping
• Sample data initialization with Japanese user data
• Repository pattern for data access

Example Usage

# Generate JWT token
curl -X POST "http://localhost:8080/auth/token" \
  -H "Content-Type: application/json" \
  -d '{"username": "testuser", "role": "admin"}'

# Retrieve user data
curl -X GET "http://localhost:8080/api/users/1" \
  -H "Authorization: Bearer <JWT_TOKEN>"

# Response:
{
  "id": 1,
  "name": "田中 太郎",
  "email": "tanaka@example.com", 
  "role": "admin"
}

Testing

The implementation includes comprehensive test coverage:

• Successful user retrieval with valid authentication
• User not found scenarios (404 response)
• Authentication failures (401 response)
• Input validation for invalid ID formats (400 response)
• Negative ID validation (400 response)
• Invalid JWT token handling (401 response)

All tests pass and validate the exact behavior specified in the requirements.

Technology Stack

• Spring Boot 3.2.0 - Main application framework
• Spring Security - Authentication and authorization
• Spring Data JPA - Database access layer
• H2 Database - In-memory database for demo
• JJWT - JWT token implementation
• JUnit 5 - Testing framework

The implementation follows Spring Boot best practices with proper separation of concerns, comprehensive error handling, and production-ready security configuration.

Fixes #5.

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