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/config.py

@@ -112,6 +112,28 @@ class Settings(BaseSettings):
         description="Quick links displayed as icons in the header",
     )
 
+    # Audio visualizer
+    visualizer_enabled: bool = Field(
+        default=True,
+        description="Enable audio spectrum visualizer (requires soundcard + numpy)",
+    )
+    visualizer_fps: int = Field(
+        default=25,
+        description="Visualizer update rate in frames per second",
+        ge=10,
+        le=60,
+    )
+    visualizer_bins: int = Field(
+        default=32,
+        description="Number of frequency bins for the visualizer",
+        ge=8,
+        le=128,
+    )
+    visualizer_device: Optional[str] = Field(
+        default=None,
+        description="Loopback audio device name for visualizer (None = auto-detect)",
+    )
+
     @classmethod
     def load_from_yaml(cls, path: Optional[Path] = None) -> "Settings":
         """Load settings from a YAML configuration file."""