feat(account): /account/update_region_code (accept-only)

SVSim.EmulatedEntrypoint/Controllers/AccountController.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
 using SVSim.Database;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Account;
 
 namespace SVSim.EmulatedEntrypoint.Controllers;
@@ -46,4 +47,14 @@ public class AccountController : SVSimController
         // so the translation middleware emits the right msgpack shape.
         return Ok(Array.Empty<object>());
     }
+
+    [HttpPost("update_region_code")]
+    public ActionResult<EmptyResponse> UpdateRegionCode([FromBody] AccountUpdateRegionCodeRequest _)
+    {
+        if (!TryGetViewerId(out long __)) return Unauthorized();
+        // No-op: actual region value is sent in the REGION_CODE HTTP header, which we
+        // don't currently consume server-side. The body carries only initialize_flag,
+        // which is a first-set-vs-update signal we have no use for yet.
+        return new EmptyResponse();
+    }
 }