feat(viewer): add MyPageBgSelectType + MyPageBgId scalars + MyPageBgRotation owned collection

Adds BGType persistence (0=Deck/1=CustomBG/2=RandomBG) to Viewer via two scalar
columns and an owned collection keyed (ViewerId, Slot). Two persistence tests
confirm round-trip and zero-defaults on fresh viewers.

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

SVSim.Database/Models/MyPageBgRotationEntry.cs

@@ -0,0 +1,16 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row per (viewer, slot) in the viewer's saved MyPage BG rotation pool. The client posts
+/// the full pool on every <c>/user_mypage/update</c> regardless of mode, so the server overwrites
+/// it atomically each time. Slot is the 0-based position; order is preserved for the
+/// <c>/mypage/index</c> echo.
+/// </summary>
+[Owned]
+public class MyPageBgRotationEntry
+{
+    public int Slot { get; set; }
+    public int BgId { get; set; }
+}

SVSim.Database/Models/Viewer.cs

@@ -33,6 +33,12 @@ public class Viewer : BaseEntity<long>
     
     public DateTime LastLogin { get; set; }
 
+    /// <summary>BGType enum: 0=Deck, 1=CustomBG, 2=RandomBG. Default 0 = follow equipped deck's leader skin.</summary>
+    public int MyPageBgSelectType { get; set; }
+
+    /// <summary>The single chosen MyPageBG cosmetic id, used when SelectType=CustomBG. 0 = none.</summary>
+    public int MyPageBgId { get; set; }
+
     #region Owned
 
     public ViewerInfo Info { get; set; } = new ViewerInfo();
@@ -67,6 +73,8 @@ public class Viewer : BaseEntity<long>
 
     public List<ViewerFreePackClaim> FreePackClaims { get; set; } = new List<ViewerFreePackClaim>();
 
+    public List<MyPageBgRotationEntry> MyPageBgRotation { get; set; } = new List<MyPageBgRotationEntry>();
+
     public List<ViewerGachaPointBalance> GachaPointBalances { get; set; } = new List<ViewerGachaPointBalance>();
 
     public List<ViewerGachaPointReceived> GachaPointReceived { get; set; } = new List<ViewerGachaPointReceived>();

SVSim.Database/SVSimDbContext.cs

@@ -172,6 +172,12 @@ public class SVSimDbContext : DbContext
             b.HasKey("ViewerId", nameof(ViewerFreePackClaim.FreeGachaCampaignId));
             b.Property(x => x.FreeGachaCampaignId).ValueGeneratedNever();
         });
+        modelBuilder.Entity<Viewer>().OwnsMany(v => v.MyPageBgRotation, b =>
+        {
+            b.WithOwner().HasForeignKey("ViewerId");
+            b.HasKey("ViewerId", nameof(MyPageBgRotationEntry.Slot));
+            b.Property(x => x.Slot).ValueGeneratedNever();
+        });
 
         // OwnedCardEntry and OwnedItemEntry use composite PK (ViewerId, Id) where Id is auto-
         // generated, which silently permits multiple rows per (Viewer, Card) or (Viewer, Item).

SVSim.UnitTests/Persistence/MyPageBgPersistenceTests.cs

@@ -0,0 +1,61 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database;
+using SVSim.Database.Models;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Persistence;
+
+public class MyPageBgPersistenceTests
+{
+    [Test]
+    public async Task Viewer_round_trips_mypage_bg_scalars_and_rotation_pool()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var ctx = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            var viewer = await ctx.Viewers
+                .Include(v => v.MyPageBgRotation)
+                .FirstAsync(v => v.Id == viewerId);
+            viewer.MyPageBgSelectType = 2;
+            viewer.MyPageBgId = 1213410310;
+            viewer.MyPageBgRotation.Add(new MyPageBgRotationEntry { Slot = 0, BgId = 1211410310 });
+            viewer.MyPageBgRotation.Add(new MyPageBgRotationEntry { Slot = 1, BgId = 1212410310 });
+            viewer.MyPageBgRotation.Add(new MyPageBgRotationEntry { Slot = 2, BgId = 1213410310 });
+            await ctx.SaveChangesAsync();
+        }
+
+        using var verifyScope = factory.Services.CreateScope();
+        var ctx2 = verifyScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        var roundtrip = await ctx2.Viewers
+            .Include(v => v.MyPageBgRotation)
+            .AsNoTracking()
+            .FirstAsync(v => v.Id == viewerId);
+
+        Assert.That(roundtrip.MyPageBgSelectType, Is.EqualTo(2));
+        Assert.That(roundtrip.MyPageBgId, Is.EqualTo(1213410310));
+        Assert.That(roundtrip.MyPageBgRotation.OrderBy(r => r.Slot).Select(r => r.BgId),
+            Is.EqualTo(new[] { 1211410310, 1212410310, 1213410310 }));
+    }
+
+    [Test]
+    public async Task Viewer_fresh_viewer_has_zero_defaults_and_empty_rotation()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+
+        using var scope = factory.Services.CreateScope();
+        var ctx = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        var viewer = await ctx.Viewers
+            .Include(v => v.MyPageBgRotation)
+            .AsNoTracking()
+            .FirstAsync(v => v.Id == viewerId);
+
+        Assert.That(viewer.MyPageBgSelectType, Is.EqualTo(0));
+        Assert.That(viewer.MyPageBgId, Is.EqualTo(0));
+        Assert.That(viewer.MyPageBgRotation, Is.Empty);
+    }
+}