Prebuilt deck purchasing and fixes

SVSim.Database/Models/ViewerBuildDeckProductPurchase.cs

@@ -0,0 +1,14 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// Per-viewer, per-product purchase counter. Owned collection on Viewer.
+/// Unique (ViewerId, ProductId) enforced in SVSimDbContext per project_owned_collection_unique_index.
+/// </summary>
+[Owned]
+public class ViewerBuildDeckProductPurchase
+{
+    public int ProductId { get; set; }
+    public int PurchaseCount { get; set; }
+}