Add audio visualizer with spectrogram, beat-reactive art, and device selection

- New audio_analyzer service: loopback capture via soundcard + numpy FFT
- Real-time spectrogram bars below album art with accent color gradient
- Album art and vinyl pulse to bass energy beats
- WebSocket subscriber pattern for opt-in audio data streaming
- Audio device selection in Settings tab with auto-detect fallback
- Optimized FFT pipeline: vectorized cumsum bin grouping, pre-serialized JSON broadcast
- Visualizer config: enabled/fps/bins/device in config.yaml
- Optional deps: soundcard + numpy (graceful degradation if missing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

media_server/routes/media.py

@@ -268,6 +268,59 @@ async def get_artwork(_: str = Depends(verify_token_or_query)) -> Response:
     return Response(content=art_bytes, media_type=content_type)
 
 
+@router.get("/visualizer/status")
+async def visualizer_status(_: str = Depends(verify_token)) -> dict:
+    """Check if audio visualizer is available and running."""
+    from ..services.audio_analyzer import get_audio_analyzer
+
+    analyzer = get_audio_analyzer()
+    return {
+        "available": analyzer.available,
+        "running": analyzer.running,
+        "current_device": analyzer.current_device,
+    }
+
+
+@router.get("/visualizer/devices")
+async def visualizer_devices(_: str = Depends(verify_token)) -> list[dict[str, str]]:
+    """List available loopback audio devices for the visualizer."""
+    from ..services.audio_analyzer import AudioAnalyzer
+
+    loop = asyncio.get_event_loop()
+    return await loop.run_in_executor(None, AudioAnalyzer.list_loopback_devices)
+
+
+@router.post("/visualizer/device")
+async def set_visualizer_device(
+    request: dict,
+    _: str = Depends(verify_token),
+) -> dict:
+    """Set the loopback audio device for the visualizer.
+
+    Body: {"device_name": "Device Name" | null}
+    Passing null resets to auto-detect.
+    """
+    from ..services.audio_analyzer import get_audio_analyzer
+
+    device_name = request.get("device_name")
+    analyzer = get_audio_analyzer()
+
+    # Restart with new device
+    was_running = analyzer.running
+    success = analyzer.set_device(device_name)
+
+    # Restart audio broadcast if needed
+    if was_running and success and analyzer.running:
+        await ws_manager.stop_audio_monitor()
+        await ws_manager.start_audio_monitor(analyzer)
+
+    return {
+        "success": success,
+        "current_device": analyzer.current_device,
+        "running": analyzer.running,
+    }
+
+
 @router.websocket("/ws")
 async def websocket_endpoint(
     websocket: WebSocket,
@@ -321,6 +374,10 @@ async def websocket_endpoint(
                 if volume is not None:
                     controller = get_media_controller()
                     await controller.set_volume(int(volume))
+            elif data.get("type") == "enable_visualizer":
+                await ws_manager.subscribe_visualizer(websocket)
+            elif data.get("type") == "disable_visualizer":
+                await ws_manager.unsubscribe_visualizer(websocket)
 
     except WebSocketDisconnect:
         await ws_manager.disconnect(websocket)