- Updated src/server/api/websocket.py docstrings to document key as primary series identifier
- Updated src/server/models/websocket.py with detailed docstrings explaining key and folder fields in message payloads
- Updated src/server/services/websocket_service.py broadcast method docstrings to document key field usage
- Added WebSocket message example with key in infrastructure.md
- All 83 WebSocket tests pass
- Task 4.4 marked as complete in instructions.md