🐛 | Fix SonarQube issues across codebase

- Add docstrings to all empty Qt stub methods (S1186)
- Fix BLOCKER S1845: rename playbackState to get_playback_state
- Rename enum fields to snake_case with CamelCase aliases (S116)
- Rename stub methods to snake_case (set_source, set_position, etc.) (S100)
- Rename unused timeout param to msecs (S1172)
- Remove unused state variables in tests (S1481)
- Replace 'is' with '==' for enum comparisons (S5795)
- Remove unnecessary list() calls (S7504)
- Rename QTh to qt_cls (S117)
- Remove commented-out code blocks (S125)
- Add docstring to FallbackServiceInterface.emit_properties_changed (S1186)

mpris.py

@@ -52,7 +52,7 @@ except ImportError:
         def __init__(self, name: str) -> None:
             self._interface_name = name
         def emit_properties_changed(self, changed, invalidated) -> None:
-            pass
+            """Stub — no-op; real implementation emits D-Bus PropertiesChanged."""
 
     ServiceInterface = _FallbackServiceInterface  # type: ignore[assignment,misc]
     dbus_method = lambda **kw: lambda f: f