more audit work

src/Signals/StellaOps.Signals/Scm/Webhooks/GitHubEventMapper.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Text.Json;
 using StellaOps.Signals.Scm.Models;
 
@@ -53,7 +54,7 @@ public sealed class GitHubEventMapper : IScmEventMapper
             EventId = deliveryId,
             Provider = ScmProvider.GitHub,
             EventType = scmEventType,
-            Timestamp = DateTimeOffset.UtcNow,
+            Timestamp = ExtractTimestamp(payload),
             Repository = repository,
             Actor = ExtractActor(payload),
             Ref = refName ?? GetString(payload, "ref"),
@@ -266,6 +267,36 @@ public sealed class GitHubEventMapper : IScmEventMapper
         };
     }
 
+    private static DateTimeOffset ExtractTimestamp(JsonElement payload)
+    {
+        var timestamp = GetNestedString(payload, "head_commit", "timestamp")
+            ?? GetNestedString(payload, "workflow_run", "updated_at")
+            ?? GetNestedString(payload, "workflow_run", "created_at")
+            ?? GetNestedString(payload, "pull_request", "updated_at")
+            ?? GetNestedString(payload, "pull_request", "created_at")
+            ?? GetNestedString(payload, "release", "published_at")
+            ?? GetNestedString(payload, "check_run", "completed_at")
+            ?? GetNestedString(payload, "check_run", "started_at");
+
+        return TryParseTimestamp(timestamp) ?? DateTimeOffset.UnixEpoch;
+    }
+
+    private static DateTimeOffset? TryParseTimestamp(string? value)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            return null;
+        }
+
+        return DateTimeOffset.TryParse(
+            value,
+            CultureInfo.InvariantCulture,
+            DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal,
+            out var parsed)
+            ? parsed
+            : null;
+    }
+
     private static string? GetString(JsonElement element, string property)
     {
         return element.TryGetProperty(property, out var value) && value.ValueKind == JsonValueKind.String