feat(viewer): migration for mypage bg selection

Adds AddViewerMyPageBgSelection migration: two int scalars on Viewers
(MyPageBgId, MyPageBgSelectType default 0) and ViewerMyPageBgRotation
owned table with composite PK (ViewerId, Slot), FK cascade to Viewers.
Also adds ToTable(ViewerMyPageBgRotation) to OwnsMany config so EF
uses the correct table name instead of defaulting to the entity class.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/SVSimDbContext.cs

@@ -174,6 +174,7 @@ public class SVSimDbContext : DbContext
         });
         modelBuilder.Entity<Viewer>().OwnsMany(v => v.MyPageBgRotation, b =>
         {
+            b.ToTable("ViewerMyPageBgRotation");
             b.WithOwner().HasForeignKey("ViewerId");
             b.HasKey("ViewerId", nameof(MyPageBgRotationEntry.Slot));
             b.Property(x => x.Slot).ValueGeneratedNever();