add .env file support for application settings

[DogukanUrker]

Summary

• integrate python-dotenv to load environment variables from a .env file
• all settings in Settings class now read from environment variables with original hardcoded values preserved as defaults
• add .env.example as a reference for all available configuration options

Changes

• app/settings.py - uses load_dotenv() + os.environ.get() with defaults for every setting. added _bool() helper for parsing string booleans.
• app/pyproject.toml - added python-dotenv dependency
• .env.example - new file documenting all configurable env vars

Test plan

• verify app starts normally without a .env file (defaults should work as before)
• create a .env with overridden values (e.g. APP_PORT=5000) and verify they take effect
• verify boolean settings parse correctly (True/False/1/0/yes/no)
• run make test to confirm E2E tests pass

fixes #225