server: infer archive collected_at from source events

internal/server/handlers.go

@@ -1589,7 +1589,29 @@ func applyArchiveSourceMetadata(result *models.AnalysisResult) {
 	result.SourceType = models.SourceTypeArchive
 	result.Protocol = ""
 	result.TargetHost = ""
-	result.CollectedAt = time.Now().UTC()
+	if result.CollectedAt.IsZero() {
+		result.CollectedAt = inferArchiveCollectedAt(result)
+	}
+}
+
+func inferArchiveCollectedAt(result *models.AnalysisResult) time.Time {
+	if result == nil {
+		return time.Now().UTC()
+	}
+
+	var latest time.Time
+	for _, event := range result.Events {
+		if event.Timestamp.IsZero() {
+			continue
+		}
+		if latest.IsZero() || event.Timestamp.After(latest) {
+			latest = event.Timestamp
+		}
+	}
+	if !latest.IsZero() {
+		return latest.UTC()
+	}
+	return time.Now().UTC()
 }
 
 func applyCollectSourceMetadata(result *models.AnalysisResult, req CollectRequest) {