Implement comprehensive REST API backend for Java Fundamentals applications

[Copilot]

This PR transforms the existing collection of 70+ standalone Java console applications into a modern, comprehensive REST API backend using Spring Boot. The implementation provides a unified service architecture that exposes all the fundamental programming exercises through well-designed API endpoints.

What's Changed

Backend Architecture

• Spring Boot 3.2.0 application with Maven build configuration
• REST API endpoints with JSON request/response format
• H2 database integration for persistent storage of execution history
• Comprehensive input validation and error handling
• Modular executor pattern for easy extensibility

Core Applications Implemented

The backend now provides API access to 6 key applications:

1. Population Projection - Calculate population growth over time based on birth, death, and immigration rates
2. Financial Application - Compound interest calculations with monthly deposits
3. Ticket Price Calculator - Age-based pricing logic (under 16: $10, 16+: $20)
4. Loan Calculator - Monthly payment and total cost calculations
5. Lottery Game - Two-digit lottery with different winning levels and prizes
6. Triangle Angles - Calculate all three angles from triangle coordinates

API Endpoints

• GET /api/v1/ - API information and available endpoints
• GET /api/v1/applications - List all available applications with descriptions
• POST /api/v1/execute - Execute any application with input parameters
• GET /api/v1/history - Retrieve execution history with pagination
• GET /api/v1/result/{id} - Get specific execution result by ID
• GET /api/v1/health - Health check endpoint

Interactive Web Frontend

• Modern responsive UI built with HTML5, CSS3, and vanilla JavaScript
• Dynamic form generation based on selected application type
• Real-time execution and result display
• Execution history tracking with automatic refresh
• Comprehensive error handling and user feedback

Database Integration

• JPA entities for calculation results storage
• Repository layer with custom query methods
• Automatic schema creation and data persistence
• H2 console access for development and debugging

Testing & Quality

• Unit tests for core application executors
• Input validation with meaningful error messages
• Comprehensive logging for monitoring and debugging
• Maven build lifecycle integration

Example Usage

# Get available applications
curl -X GET http://localhost:8080/api/v1/applications

# Execute population projection
curl -X POST http://localhost:8080/api/v1/execute \
  -H "Content-Type: application/json" \
  -d '{
    "applicationType": "population-projection",
    "inputData": {"years": 5}
  }'

Live Demo

Once running, the backend provides:

• API Base: http://localhost:8080/api/v1/
• Interactive Frontend: http://localhost:8080/api/
• Database Console: http://localhost:8080/api/h2-console

Technical Benefits

• Unified Architecture: Consolidates all standalone applications into a cohesive service
• REST API Standard: Follows REST principles with proper HTTP methods and status codes
• Scalable Design: Modular executor pattern allows easy addition of new applications
• Professional Grade: Includes validation, error handling, logging, and persistence
• Developer Friendly: Complete documentation, interactive frontend, and database console

This implementation transforms the educational Java exercises into a production-ready backend service while maintaining all the original functionality and adding significant value through modern web architecture patterns.

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