feat: add dependency injection container

[allisson]

Introduce a custom DI container in internal/app/ to centralize all application component initialization and wiring, following Clean Architecture principles and improving maintainability.

• internal/app/di.go: Complete DI container implementation with lazy initialization, singleton pattern, and clean resource management

• internal/app/di_test.go: Comprehensive test suite with 6 test cases

• internal/app/README.md: Detailed documentation with examples and guides

• REFACTORING_SUMMARY.md: Complete summary of refactoring changes

• cmd/app/main.go: Simplified from 236 to 182 lines (-23%)

  • Removed manual dependency wiring from runServer() and runWorker()
  • Removed setupLogger() function (moved to container)
  • Replaced closeDB() with closeContainer()
  • All commands now use container pattern

• README.md: Updated project documentation

  • Added DI container to features list
  • Updated project structure to include internal/app/
  • Added comprehensive DI container documentation section
  • Enhanced "Adding New Domains" guide with DI registration steps

• Lazy Initialization: Components created only on first access

• Singleton Pattern: Each component initialized once and reused

• Thread-Safe: Using sync.Once for concurrent access safety

• Error Handling: Comprehensive error propagation and storage

• Resource Cleanup: Unified Shutdown() method for all resources

• Testability: Easily testable and mockable for integration tests

1. Centralized dependency management - all wiring in one place
2. Simplified main.go - reduced complexity by 23%
3. Eliminated duplicate initialization code across commands
4. Improved testability with mockable container
5. Better scalability - easy to add new domains
6. Clean resource management with unified shutdown
7. No external DI framework dependencies