Full rearchitecture using Claude

- event bus systen
- all components are independent
- preparation for web frontend

python-backend/notebooks/README.md

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+# Notebooks
+
+Exploratory material, **not part of the running backend** and not imported by it.
+
+`audio_analysis.py` and the three `C5S*` notebooks are chroma/chord-recognition course
+work (they still reference stale absolute paths). `effect_debug.ipynb` is scratch work
+for tinkering with LED effects.
+
+They need `librosa`, `numba` and `numpy`, which are deliberately not in the backend's
+dependencies:
+
+```sh
+pip install librosa numba numpy jupyter
+```