feat: add player audio effects module with real-time pipeline, distortion, fourth density, and UI controls

tests/test_api_surface.py

@@ -21,6 +21,7 @@ EXPECTED_ENDPOINTS = {
     ("post", "/api/stop"),
     ("post", "/api/volume"),
     ("post", "/api/volume_boost"),
+    ("post", "/api/effects/toggle"),
     ("post", "/api/loop"),
     ("post", "/api/loop_playlist"),
     ("post", "/api/loop_cycle"),