feat(account): /account/update_birth persists viewer birth date

Strict yyyy-MM-dd parse (matches /load/index round-trip format) — rejects
malformed input. data_headers.servertime is emitted by the standard
envelope, which is what the client reads into BirthDayUpdateServerTime.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/AccountController.cs

@@ -57,4 +57,24 @@ public class AccountController : SVSimController
         // which is a first-set-vs-update signal we have no use for yet.
         return new EmptyResponse();
     }
+
+    [HttpPost("update_birth")]
+    public async Task<ActionResult<EmptyResponse>> UpdateBirth([FromBody] AccountUpdateBirthRequest request)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+        // Wire format is "yyyy-MM-dd" (see /load/index UserInfo.Birthday round-trip).
+        // Parse strict; client only ever submits via its date-picker dialog.
+        if (!DateTime.TryParseExact(request.Birth, "yyyy-MM-dd",
+                System.Globalization.CultureInfo.InvariantCulture,
+                System.Globalization.DateTimeStyles.AssumeUniversal | System.Globalization.DateTimeStyles.AdjustToUniversal,
+                out var birth))
+            return BadRequest(new { error = "birth_invalid" });
+
+        var viewer = await _db.Viewers.FirstAsync(v => v.Id == viewerId);
+        viewer.Info.BirthDate = birth;
+        await _db.SaveChangesAsync();
+        // data_headers.servertime drives the client's BirthDayUpdateServerTime — the standard
+        // envelope already emits it, so an empty data payload is sufficient.
+        return new EmptyResponse();
+    }
 }

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Account/AccountUpdateBirthRequest.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Account;
+
+[MessagePackObject]
+public class AccountUpdateBirthRequest : BaseRequest
+{
+    [JsonPropertyName("birth")]
+    [Key("birth")]
+    public string Birth { get; set; } = string.Empty;
+}