Audit error codes (mpbx-audit)

Codes raised by mpbx_audit (W3-AUD-01): the ClickHouse audit relay and the audit reader. Kinds map to HTTP statuses in the API layer as for the common codes. Messages never contain row values. The relay and the events producers also raise the MPBX-EVENTS-* codes of events.md (MPBX-EVENTS-SYSTEM-SCOPE-REQUIRED when an install-wide record is written on anything but a system transaction, MPBX-EVENTS-TENANT-MISMATCH for a record written in another company's transaction) and the MPBX-CH-* codes of storage.md. Guide: docs/guides/audit.md.

Code	Kind	Meaning	What to do
MPBX-AUDIT-UNREADABLE	internal	ClickHouse returned an audit_log row that does not have the expected shape (missing column, unparsable id or time). Nothing was returned for that request.	Report it with the log line: the table and the reader are out of step (an unapplied ClickHouse migration is the usual cause; run mpbx migrate --ch).
MPBX-API-NOT-CONFIGURED	unavailable	The audit routes answer 503 with this code when the server runs without clickhouse.url. The audit records are not lost: they wait in the Postgres outbox and are relayed once ClickHouse is configured.	Set [clickhouse] url and restart.