feat: add worker domain events and processing metrics

[AKogut]

Closes #102

Summary

Closes the #17 scope items that were never built: the worker emitted no domain events for downstream stages and exposed no processing metrics.

Changes

• Domain events (events.ts): a small typed bus emitting run.processed, identity.created, identity.moved, and score.updated. Identity events are collected during the transaction and emitted after it commits, so subscribers never observe uncommitted state. A throwing subscriber is isolated and reported rather than failing the job. This is the seam downstream stages (signature clustering, AI RCA) will subscribe to.
• Metrics (telemetry.ts): jobs_processed / jobs_failed / jobs_dead_lettered counters, processing_lag (enqueue → pickup) and processing_duration histograms, and a queue_depth observable gauge. All no-op until FLAKEMETRY_SELF_OTEL_ENDPOINT registers an OTLP metrics exporter, matching the API's self-instrumentation.
• queue.dequeue now returns createdAt — required to compute processing lag.

Testing

• worker: 9 tests (up from 3) — event bus unit tests (delivery, per-event isolation, unsubscribe, throwing subscriber) and processor tests asserting emitted identity.created / score.updated / run.processed plus identity.moved on a file move
• full turbo pipeline green, 38/38 tasks
• worker env vars documented in docs/configuration.md