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

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

SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using SVSim.Database;
+using SVSim.Database.Models;
+using SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+namespace SVSim.EmulatedEntrypoint.Controllers;
+
+/// <summary>
+/// /user_mypage/* — viewer-scoped MyPage configuration writes. Separate from the
+/// <c>/mypage/*</c> family because the wire URL family is distinct.
+/// </summary>
+[Route("user_mypage")]
+public sealed class UserMyPageController : SVSimController
+{
+    private readonly SVSimDbContext _db;
+
+    public UserMyPageController(SVSimDbContext db) => _db = db;
+
+    [HttpPost("update")]
+    public async Task<ActionResult<UserMyPageUpdateResponse>> Update(
+        [FromBody] UserMyPageUpdateRequest request,
+        CancellationToken ct)
+    {
+        if (!TryGetViewerId(out var viewerId)) return Unauthorized();
+
+        var viewer = await _db.Viewers
+            .Include(v => v.MyPageBgRotation)
+            .FirstOrDefaultAsync(v => v.Id == viewerId, ct);
+        if (viewer is null) return NotFound();
+
+        viewer.MyPageBgSelectType = request.SelectType;
+        viewer.MyPageBgId = ParseIdOrZero(request.MyPageId);
+
+        viewer.MyPageBgRotation.Clear();
+        for (int slot = 0; slot < request.MyPageIdList.Count; slot++)
+        {
+            viewer.MyPageBgRotation.Add(new MyPageBgRotationEntry
+            {
+                Slot = slot,
+                BgId = ParseIdOrZero(request.MyPageIdList[slot]),
+            });
+        }
+
+        await _db.SaveChangesAsync(ct);
+        return new UserMyPageUpdateResponse();
+    }
+
+    private static int ParseIdOrZero(string s) =>
+        int.TryParse(s, out var n) ? n : 0;
+}

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

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+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
+{
+}