Get comprehensive audit trail for a document -- completed or otherwise terminal (voided, declined, expired, ...).

Completed documents have a persisted CompletedDocument + DocumentAuditTrail pair (written by DocumentCompletionService at completion time) and are served from that pair, unchanged. A document whose envelope was voided (or otherwise ended without completing) never produces a CompletedDocument -- there is no signed PDF and no document hash for a completion certificate -- so that case falls back to looking the SigningSession up directly by document_id and building its audit trail on the fly, uncached, with no completion certificate.

Args: document_id: Completed document ID db: Database session

Returns: Audit trail with all events and completion certificate