fix: KrxDataService HTTPS protocol + Accept headers

- Changed: http:// → https://data-dbg.krx.co.kr
- Added: Accept: application/json header
- Added: Content-Type: application/json; charset=utf-8 header
- Result: HTTP 200 OK (verified with real KRX API)

KRX API now fully functional. Response includes OutBlock_1 with real stock data:
- ISU_CD (stock code)
- ISU_NM (stock name)
- TDD_CLSPRC (closing price)
- ACC_TRDVOL (trading volume)
- Plus: Open/High/Low prices, market cap

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Modules.ModelOperations/ShadowRun/Services/KrxDataService.cs

@@ -19,7 +19,7 @@ public sealed class KrxDataService : IKrxDataService
     private const int MaxRetries = 3;
     private const int InitialBackoffMs = 100;
     private const int MaxBackoffMs = 30000;
-    private const string KrxApiBaseUrl = "http://data-dbg.krx.co.kr";  // Stock price API (from pykrx-openapi)
+    private const string KrxApiBaseUrl = "https://data-dbg.krx.co.kr";  // Stock price API (HTTPS, from pykrx-openapi)
     private const string KrxApiEndpoint = "/svc/apis/sto/stk_bydd_trd";  // KOSPI daily trading endpoint
 
     private static readonly Action<ILogger, string, DateOnly, DateOnly, Exception?> LogFetchingOhlcv =
@@ -187,6 +187,8 @@ public sealed class KrxDataService : IKrxDataService
             {
                 var request = new HttpRequestMessage(HttpMethod.Get, endpoint);
                 request.Headers.Add("AUTH_KEY", apiKey);
+                request.Headers.Add("Accept", "application/json");
+                request.Content = new StringContent("", System.Text.Encoding.UTF8, "application/json; charset=utf-8");
 
                 var response = await _httpClient.SendAsync(request, cancellationToken);