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

frontend/src/components/DetailPanel.tsx

@@ -141,6 +141,17 @@ export default function DetailPanel({ clip, onClose, onRename, onDelete, onUploa
             />
           </div>
 
+          {/* FFT Waveform */}
+          {clip.status === 'ready' && (
+            <div className="rounded-lg overflow-hidden">
+              <img
+                src={`/api/clips/${clip.id}/waveform`}
+                alt="Waveform"
+                className="w-full h-20 object-cover"
+              />
+            </div>
+          )}
+
           {/* Title */}
           <div>
             <div className="flex items-center justify-between mb-1">