Wires MyPageController.Index to read Viewer.MyPageBgId/SelectType/BgRotation
instead of emitting an empty MyPageBgSetting placeholder; adds Include for
MyPageBgRotation in ViewerRepository.GetViewerByShortUdid; adds fresh-viewer
zero-defaults test and write→read round-trip test (9/9 controller tests pass).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>