Add TensorBoard logging; backfill script for stdout logs
- report_to=["tensorboard"] for live training run metrics
- Add tensorboard>=2.20.0 and setuptools==81.0.0 deps (setuptools 82
removed pkg_resources which tensorboard 2.20 still imports)
- src/backfill_tb.py: parse stdout training log and write TB events
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>