feat(user-mypage): UserMyPageController + DTOs + persistence tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateRequest.cs

@@ -0,0 +1,28 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+/// <summary>
+/// Body of <c>POST /user_mypage/update</c>. Client task: <c>MyPageSettingUpdateTask</c>
+/// (Shadowverse_Code_2026-05-23/Wizard/MyPageSettingUpdateTask.cs). Note that
+/// <c>select_type</c> is the only int on the wire — id fields are strings.
+/// </summary>
+[MessagePackObject]
+public sealed class UserMyPageUpdateRequest
+{
+    /// <summary>BGType enum: 0=Deck, 1=CustomBG, 2=RandomBG. Client sends as an int.</summary>
+    [JsonPropertyName("select_type")]
+    [Key("select_type")]
+    public int SelectType { get; set; }
+
+    /// <summary>Chosen BG id when SelectType=CustomBG; empty or "0" otherwise.</summary>
+    [JsonPropertyName("mypage_id")]
+    [Key("mypage_id")]
+    public string MyPageId { get; set; } = "0";
+
+    /// <summary>Saved rotation pool, in slot order; client sends the full list on every call.</summary>
+    [JsonPropertyName("mypage_id_list")]
+    [Key("mypage_id_list")]
+    public List<string> MyPageIdList { get; set; } = new();
+}

SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateResponse.cs

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+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+/// <summary>
+/// Empty response payload. The client's <c>MyPageSettingUpdateTask.Parse()</c> is the default
+/// pass-through; server just acknowledges.
+/// </summary>
+[MessagePackObject(true)]
+public sealed class UserMyPageUpdateResponse
+{
+}