fix(notebooks): add Redis flush to clear old checkpoints

Add Redis flush command at the start of notebooks to clear old checkpoints
that were created with the buggy serializer (before dataclass support was added).

This is necessary because:
1. Jupyter kernels cache Python modules
2. Old checkpoints in Redis were serialized without dataclass support
3. Deserializing old checkpoints causes AttributeError even with fixed code

The flush ensures notebooks start fresh with the updated serializer.

Author: bsbodden