feat(06-02): add feature extraction and segmentation backend modules + API

- Create lightsync/audio/features.py with extract_features and extract_features_async
- Create lightsync/audio/segments.py with detect_segments via agglomerative clustering
- Add /api/audio/segments and /api/audio/features endpoints to audio.py
- Initialize app.state.segments cache in main.py lifespan

lightsync/main.py

@@ -25,6 +25,8 @@ async def lifespan(app: FastAPI):
 
     # Beat analysis cache: filepath -> {"tempo": float, "beats": [float, ...]}
     app.state.beats = {}
+    # Segments cache: "filepath:n_segments" -> [{start, end, label}, ...]
+    app.state.segments = {}
 
     # UDP sender for device communication (Phase 3)
     udp_sender = UDPSender()