Add marker for auditable models without owner

Models implementing AuditableWithoutOwner intentionally have no audit
owner, either because they belong to neither an organization nor a user,
or because they and their audits have to be kept when the owner is
deleted (for example billing records). Adds helpers and a scope on the
audit model to find audits that are missing an owner although they
should have one.

app/Models/Concerns/AuditableWithoutOwner.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models\Concerns;
+
+/**
+ * Marks an auditable model whose audits intentionally have no owner (neither an organization nor a user),
+ * either because the model belongs to neither of them, or because the model and its audits have to be kept
+ * when its organization or user is deleted (for example billing records).
+ * Audits of these models are not deleted together with an organization or user,
+ * and are not considered as missing an owner (for example by the audit backfill command).
+ */
+interface AuditableWithoutOwner {}