Prebuilt deck purchasing and fixes

SVSim.Database/Models/BuildDeckSeriesRewardEntry.cs

@@ -0,0 +1,20 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One tier-reward item attached to a prebuilt-deck series. Owned by BuildDeckSeriesEntry.
+/// Wire shape: flattened from /build_deck/info's `series_rewards` dict — each tier (keyed
+/// by total-purchases-from-series threshold) carries a list of rewards; this row is one
+/// (TierIndex, ItemIndex) cell.
+/// </summary>
+[Owned]
+public class BuildDeckSeriesRewardEntry
+{
+    public int TierIndex { get; set; }   // 1, 2, 3, ... — unlock threshold
+    public int ItemIndex { get; set; }   // ordinal within tier
+    public int RewardType { get; set; }
+    public long RewardDetailId { get; set; }
+    public int RewardNumber { get; set; }
+    public int MessageId { get; set; }
+}