V13-FE-006: consolidate approved UI and contract hardening

src/KArtSell.Host/Infrastructure/CorrelationIdMiddleware.cs

@@ -0,0 +1,34 @@
+using System.Diagnostics;
+
+namespace KArtSell.Host.Infrastructure;
+
+/// <summary>
+/// Establishes one correlation identifier at the HTTP boundary.
+/// Endpoint code may use TraceIdentifier or Items[CorrelationId], but both resolve
+/// to the same value and the response exposes it for support/replay tracing.
+/// </summary>
+public sealed class CorrelationIdMiddleware(RequestDelegate next, ILogger<CorrelationIdMiddleware> logger)
+{
+    public async Task InvokeAsync(HttpContext context)
+    {
+        var correlationId = ReadCorrelationId(context.Request.Headers["X-Correlation-Id"]);
+
+        context.TraceIdentifier = correlationId.ToString("D");
+        context.Items["CorrelationId"] = correlationId;
+        context.Response.Headers["X-Correlation-Id"] = correlationId.ToString("D");
+        Activity.Current?.SetTag("kartsell.correlation_id", correlationId.ToString("D"));
+
+        using (logger.BeginScope(new Dictionary<string, object?>
+        {
+            ["CorrelationId"] = correlationId,
+        }))
+        {
+            await next(context);
+        }
+    }
+
+    private static Guid ReadCorrelationId(string? headerValue) =>
+        Guid.TryParse(headerValue, out var correlationId) && correlationId != Guid.Empty
+            ? correlationId
+            : Guid.NewGuid();
+}