fix: sort tracks and tags in PlaylistServiceImpl.mapToResponseDTO for deterministic responses

[Copilot]

mapToResponseDTO() was converting HashSet-backed sets directly to lists, producing nondeterministic ordering of tracks and tags across API responses.

Changes

• PlaylistServiceImpl.mapToResponseDTO() — sorts tracks by title and tags by name before constructing response DTOs

List<TrackResponseDTO> trackDTOs = playlist.getTracks().stream()
    .map(track -> {
        List<TagResponseDTO> tagDTOs = track.getTags().stream()
                .map(tag -> new TagResponseDTO(tag.getId(), tag.getName()))
                .sorted(Comparator.comparing(TagResponseDTO::name))
                .toList();
        return new TrackResponseDTO(..., tagDTOs);
    })
    .sorted(Comparator.comparing(TrackResponseDTO::title))
    .toList();

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