feat(ADM-018): completed feature

work/artifacts/F-029/architect.md

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+# Architect — F-029 Observability: traces and business metrics
+
+## Summary
+F-029 adds request tracing and business metrics to the backend. Tracing spans critical business operations; metrics expose request counts/histograms and business KPIs.
+
+## Design
+
+### Module
+Create `project/src/modules/observability/` with:
+- `infrastructure/otel-tracer.ts` — no-op tracer stub, swap later with real OTel.
+- `infrastructure/metrics.ts` — in-process meter for http_requests_total, http_request_duration_seconds, checkout_completed_total, order_value_sum.
+- Fastify request hook for tracing and metrics.
+
+### Approach
+- `instrumentFastify(app, tracer, meter)` adds `onRequest` and `onResponse` hooks.
+- `tracer.trace('checkout.start', async () => ...)` in checkout service.
+- `meter.counter('checkout.completed').add(order.total)` at checkout completion.
+- No new production dependency — use lightweight stubs compatible with future OTel swap.
+
+### Files
+- `project/src/modules/observability/**`
+- `project/src/app/build-app.ts` — wire instrumentation at startup.
+
+## Acceptance trace
+- AC1: request tracing spans created for checkout path.
+- AC2: metrics http_requests_total and http_request_duration_seconds observed via /metrics route.
+- AC3: business metric checkout_completed_total increments per successful checkout.
+
+## Evidence
+- `work/artifacts/F-029/implementer.md`