Things were working, suddenly regressed

SVSim.EmulatedEntrypoint/Models/Dtos/Convention.cs

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+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos;
+
+/// <summary>
+/// Convention/offline-event participation block returned by /mypage/index.
+/// Client reads is_join_tournament, recent_start_date (null-checked, optional),
+/// and is_admin_watch_user. See MyPageTask.cs:58-63.
+/// </summary>
+[MessagePackObject]
+public class Convention
+{
+    [JsonPropertyName("is_join_tournament")]
+    [Key("is_join_tournament")]
+    public bool IsJoinTournament { get; set; }
+
+    /// <summary>
+    /// ISO datetime. Optional — omitted via WhenWritingNull when not set.
+    /// Client null-checks before parsing (MyPageTask.cs:59).
+    /// </summary>
+    [JsonPropertyName("recent_start_date")]
+    [Key("recent_start_date")]
+    public string? RecentStartDate { get; set; }
+
+    [JsonPropertyName("is_admin_watch_user")]
+    [Key("is_admin_watch_user")]
+    public bool IsAdminWatchUser { get; set; }
+}