feat: Add themed FFT waveform visualization to detail panel

Backend:
- Install ffmpeg + numpy in Docker image
- New waveform.py: FFT-based SVG generator with 64 frequency bands
- GET /api/clips/{id}/waveform — generates on demand, caches to /data/waveforms/
- DELETE /api/clips/{id} now cascade-removes waveform SVG
- Warm amber-to-orange HSL gradient (25° to 35° hue)
- Mirrored bars: top full opacity, bottom 60% opacity

Frontend:
- DetailPanel shows waveform image beneath album art (80px height)
- Only shown for clips with status='ready'

Storage: /data/waveforms/ persisted via existing ./data volume mount

backend/Dockerfile

@@ -1,5 +1,8 @@
 FROM python:3.12-slim
 
+# Install ffmpeg for audio decoding
+RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg && rm -rf /var/lib/apt/lists/*
+
 WORKDIR /app
 
 COPY requirements.txt .