refactor: implement modular architecture with DRY and SOLID principles

[iputra]

• Created modular structure with src/ directory

  • src/config: ConfigManager with Singleton pattern
  • src/core: URLGenerator, FileParser, custom exceptions
  • src/services: DownloadService, DatabaseService, GFSProcessor

• Applied DRY (Don't Repeat Yourself) principle

  • Centralized configuration management
  • No code duplication across modules
  • Reusable components

• Followed SOLID principles

  • Single Responsibility: Each class has one clear purpose
  • Open/Closed: Services support extension via dependency injection
  • Dependency Inversion: Clear layer separation

• Improvements

  • Type hints throughout codebase
  • Comprehensive docstrings
  • Constants instead of magic numbers/strings
  • Proper error handling with custom exceptions
  • Clean separation of concerns

• Added comprehensive test suite

  • test_structure.py: Structure validation (4/4 passed)
  • test_logic.py: Logic & architecture tests (7/7 passed)
  • All tests passing (11/11 - 100%)

• Documentation

  • README_REFACTORING.md: Complete guide and migration instructions
  • Inline documentation in all modules

• Backward compatibility maintained

  • Old files (utils.py, gfs-miner.py) preserved for compatibility