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

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

SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs

@@ -0,0 +1,51 @@
+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;
+}