* feat: Integrate .env file support for environment variables
feat: Add backend_example.py file support for using without a frontend

- Added .env.example with necessary environment variable placeholders: OPENAI_API_KEY, OPENAI_API_BASE, OPENAI_MODEL, SILICON_API_KEY, SILICON_MODEL.
- Updated models.py to load environment variables using python-dotenv.
- Modified gpt4 amd silicon model configurations to use values from .env file or defaults.
- Updated .gitignore to exclude .env file.
- Updated requirements.txt to include python-dotenv.
- Updated README.md to document environment variable setup and backend usage example.
- Added backend_example.py for direct backend interaction.

* (README.md): remove new features section from forked version