Audit log retention purge job

[jusso-dev]

Follow-up to #4.

The audit_log table and per-tenant organisations.audit_retention_days column (default 395) ship in #4, but the actual purge job is out of scope for that PR.

Scope

• Recurring job (Inngest / cron via instrumentation.ts) that deletes audit_log rows where created_at < now() - organisations.audit_retention_days * interval '1 day', grouped per organisation.
• Run nightly (overnight AU). Surface per-org counts via structured logs.
• Respect retention overrides — never delete rows newer than the org's configured window.
• Test: create rows then advance retention boundary to confirm purge correctness.

Acceptance

• Job is registered and runs on the schedule.
• Deletion is batched (e.g. 10k rows per statement) so it does not lock the table.
• Operator-visible metric: rows purged per org per run.