feat(db): add gacha-point balance + received tables

SVSim.Database/Models/ViewerGachaPointReceived.cs

@@ -0,0 +1,17 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// Marker row recording that a viewer has already redeemed <c>CardId</c> from <c>PackId</c>'s
+/// gacha-point exchange. Drives the per-entry <c>is_received</c> flag in
+/// <c>/pack/get_gacha_point_rewards</c>. Owned collection on <see cref="Viewer"/>.
+/// Unique index on (ViewerId, PackId, CardId) per feedback_owned_collection_unique_index.
+/// </summary>
+[Owned]
+public class ViewerGachaPointReceived
+{
+    public int PackId { get; set; }
+    public long CardId { get; set; }
+    public DateTime ReceivedAt { get; set; }
+}