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/main.py

@@ -59,8 +59,32 @@ async def lifespan(app: FastAPI):
     await ws_manager.start_status_monitor(controller.get_status)
     logger.info("WebSocket status monitor started")
 
+    # Start audio visualizer (if enabled and dependencies available)
+    analyzer = None
+    if settings.visualizer_enabled:
+        from .services.audio_analyzer import get_audio_analyzer
+
+        analyzer = get_audio_analyzer(
+            num_bins=settings.visualizer_bins,
+            target_fps=settings.visualizer_fps,
+            device_name=settings.visualizer_device,
+        )
+        if analyzer.available:
+            if analyzer.start():
+                await ws_manager.start_audio_monitor(analyzer)
+                logger.info("Audio visualizer started")
+            else:
+                logger.warning("Audio visualizer failed to start (no loopback device?)")
+        else:
+            logger.info("Audio visualizer unavailable (install soundcard + numpy)")
+
     yield
 
+    # Stop audio visualizer
+    await ws_manager.stop_audio_monitor()
+    if analyzer and analyzer.running:
+        analyzer.stop()
+
     # Stop WebSocket status monitor
     await ws_manager.stop_status_monitor()
     logger.info("Media Server shutting down")