feat(mypage): /mypage/index reflects persisted bg selection

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>

SVSim.Database/Repositories/Viewer/ViewerRepository.cs

@@ -66,6 +66,7 @@ public class ViewerRepository : IViewerRepository
             .Include(v => v.Degrees)
             .Include(v => v.LeaderSkins).ThenInclude(ls => ls.Class)
             .Include(v => v.MyPageBackgrounds)
+            .Include(v => v.MyPageBgRotation)
             .FirstOrDefaultAsync(viewer => viewer.ShortUdid == shortUdid);
     }