selfhost(observability): structured per-job audit log

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Part of #982.

Emit one structured JSON line per job lifecycle event to stdout so operators can grep, pipe to Loki, CloudWatch, Datadog, etc. without extra setup. Docker's default json-file log driver captures it automatically.

Events: job_complete, job_dead, job_error
Fields: level:audit, ts (Unix ms), job_id, payload_type, latency_ms, attempts, error

Emitted from both sqlite-queue.ts and pg-queue.ts processOne() completion paths. Full test coverage on all emit paths.

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Closing as completed (2026-07-01 issue audit): Structured per-job audit log shipped (src/selfhost/audit.ts + queue emit paths, #1549).