Websocket support, test coverage, maintenance

Added

• WebSocket support: subscribe_to_events(), subscribe_to_state_changes(), unsubscribe() with automatic reconnection
• CI pipeline via GitHub Actions (Python 3.9–3.14)
• Test coverage up to 96.6% (enforced minimum 80%)

Changed

• Minimum Python version is now 3.9
• Rewrote README with full API reference and examples

Fixed

• Various linting and type checking issues