Refactor backend to clean architecture with comprehensive testing

[rokuosan]

Summary

Complete refactor of the backend from layered architecture to clean architecture with comprehensive test coverage and CI/CD pipeline.

🏗️ Architecture Changes

• Clean Architecture: Domain → Usecase → Adapter → Infrastructure layers
• Dependency Injection: Google Wire for automatic DI container generation
• Interface-based Design: All dependencies are interfaces for better testability

🧪 Testing Implementation

• Unit Tests: All layers covered with comprehensive test suites
• Mock Repositories: Testify-based mocks for isolation
• Database Tests: sqlmock for repository layer testing
• HTTP Tests: Handler layer testing with httptest

🚀 CI/CD Pipeline

• GitHub Actions: Automated testing on push/PR
• MariaDB Service: Database container for integration tests
• Code Coverage: Built-in coverage reporting
• Code Quality: Linting, formatting, and build verification

📊 Test Results

✅ entity     - PASS (business logic tests)
✅ repository - PASS (database layer tests)  
✅ usecase    - PASS (business workflow tests)
✅ handler    - PASS (HTTP API tests)
✅ config     - PASS (configuration tests)

🔧 Key Features

• N+1 Problem Solved: Batch queries for related data
• Transaction Management: Proper transaction boundaries in usecase layer
• Error Handling: Unified error types and HTTP status mapping
• Type Safety: Strong typing throughout all layers

📦 Breaking Changes

• Complete API handler rewrite (same endpoints, new implementation)
• Database initialization moved to infrastructure layer
• Configuration system enhanced with test environment support

🛠️ Development Workflow

# Run tests locally
make test

# Run with coverage
make test-coverage

# Docker-based testing
make docker-test

# Linting
make lint

Test Plan

• All existing functionality preserved
• API endpoints maintain compatibility
• Database schema unchanged
• CI pipeline validates all changes
• No breaking changes to frontend integration

This refactor significantly improves code maintainability, testability, and development velocity while maintaining full backward compatibility.