Implements a write-behind queue with a background daemon thread in AuditService so that record() returns immediately without blocking the calling domain operation on a synchronous SQLite INSERT + COMMIT.
Changes:
- AuditService: add _writer_loop(), _write_payload(), flush() methods and a write-behind queue with a background thread
- record() enqueues the payload and returns a placeholder entry with id=-1 in async mode
- close() calls flush() to drain the queue before closing the session
- Settings: add audit_async (default True) and audit_queue_maxsize (default 10000) fields
- 20 BDD scenarios covering non-blocking record(), flush/close lifecycle, ordering, sync fallback, and settings
Closes#718
Co-authored-by: Jeffrey Phillips Freeman <the@jeffreyfreeman.me>
Co-committed-by: Jeffrey Phillips Freeman <the@jeffreyfreeman.me>
