feat: bump to 0.16.0, add checkpoint result, callbacks, and binance example

[sujitn]

Summary

• CheckpointResult type — checkpoint() now returns a CheckpointResult with checkpoint_id, replacing the old integer return
• Callback-based push subscriptions — subscribe_callback() and subscribe_stream_callback() for push-based streaming with on_data/on_error handlers
• Bump to 0.16.0 — align Python bindings version with laminardb core crate
• Binance WebSocket signals example — minimal live BTC trade signals demo using FROM WEBSOCKET connector with VWAP-based BUY/SELL/HOLD generation

Test plan

• pytest tests/test_checkpoint.py -v
• pytest tests/test_callback_subscription.py -v
• python examples/binance_signals.py (requires network access to Binance WebSocket)