🐛 | 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)

tests/test_mpris.py

@@ -217,7 +217,7 @@ class TestMprisPlayerInterface:
         assert iface.CanControl is True
 
     def test_methods_emit_via_bridge(self):
-        state, bridge, iface = self._make()
+        _, bridge, iface = self._make()
         for method_name, signal_name in [
             ("Play", "play_requested"),
             ("Pause", "pause_requested"),
@@ -231,7 +231,7 @@ class TestMprisPlayerInterface:
                 mock_sig.emit.assert_called_once()
 
     def test_seek_emits_with_offset(self):
-        state, bridge, iface = self._make()
+        _, bridge, iface = self._make()
         with mock.patch.object(bridge, "seek_requested") as mock_sig:
             iface.Seek(5000)
             mock_sig.emit.assert_called_once_with(5000)