DTOs for index mostly done, doing DB models

SVSim.EmulatedEntrypoint/Models/Dtos/SleeveIdentifier.cs

@@ -0,0 +1,16 @@
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos;
+
+/// <summary>
+/// Identifies a card sleeve.
+/// </summary>
+[MessagePackObject]
+public class SleeveIdentifier
+{
+    /// <summary>
+    /// The id of the sleeve.
+    /// </summary>
+    [Key("sleeve_id")]
+    public int SleeveId { get; set; }
+}