Add real-time audio spectrum test for audio sources and templates

- Add WebSocket endpoints for live audio spectrum streaming at ~20Hz
- Audio source test: resolves device/channel, shares stream via ref-counting
- Audio template test: includes device picker dropdown for selecting input
- Canvas-based 64-band spectrum visualizer with falling peaks and beat flash
- Channel-aware: mono sources show left/right/mixed spectrum correctly

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

server/src/wled_controller/core/processing/processor_manager.py

@@ -105,6 +105,10 @@ class ProcessorManager:
         self._metrics_history = MetricsHistory(self)
         logger.info("Processor manager initialized")
 
+    @property
+    def audio_capture_manager(self) -> AudioCaptureManager:
+        return self._audio_capture_manager
+
     @property
     def metrics_history(self) -> MetricsHistory:
         return self._metrics_history