feat(audit): sanitize stored upstream responses

backend/internal/card/validation.go

@@ -25,3 +25,11 @@ func Validate(number string) error {
 
 	return nil
 }
+
+func Mask(number string) string {
+	if Validate(number) != nil {
+		return ""
+	}
+
+	return number[:6] + "******" + number[12:]
+}