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; }
+}