feat(db): add ViewerArenaTwoPickRun entity + CandidatePair

SVSim.Database/Models/CandidatePair.cs

@@ -0,0 +1,16 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One of the 2 pick-sets offered to the player on the current draft turn. Persisted as
+/// part of <see cref="ViewerArenaTwoPickRun.PendingPickSetsJson"/>. <see cref="Id"/> is the
+/// monotonic counter the client sends back as <c>selected_id</c> on /card_choose.
+/// </summary>
+public class CandidatePair
+{
+    public long Id { get; set; }
+    public int Turn { get; set; }
+    public int SetNum { get; set; }
+    public long CardId1 { get; set; }
+    public long CardId2 { get; set; }
+    public bool IsSelected { get; set; }
+}