refactor: remove legacy volume_boost references in favor of generalized player effects API

tests/test_api_surface.py

@@ -20,7 +20,6 @@ EXPECTED_ENDPOINTS = {
     ("post", "/api/skip"),
     ("post", "/api/stop"),
     ("post", "/api/volume"),
-    ("post", "/api/volume_boost"),
     ("post", "/api/effects/toggle"),
     ("post", "/api/loop"),
     ("post", "/api/loop_playlist"),