Add YAMNet sound classification to headmic

New sound_id.py module with SoundClassifier class that runs YAMNet
(521 audio event categories) on CPU TFLite. Classifies audio every
0.5s from a ring buffer fed by the existing audio stream.

Categories: speech, alert, music, animal, household, environment, silence.
Smoothing via 20-sample history window for stable dominant category.

New endpoints: GET /sounds, GET /sounds/history
Updated: /health (sound_classification_enabled), /status (audio_scene)
Graceful degradation if model files not present.

Model download (not tracked in git):
  curl -sL 'https://tfhub.dev/google/lite-model/yamnet/classification/tflite/1?lite-format=tflite' -o models/yamnet.tflite

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -2,6 +2,9 @@
 *.ppn
 Hey-Vivi_*/
 
+# ML models (downloaded separately)
+models/*.tflite
+
 # Python
 __pycache__/
 *.py[cod]