feat(ADM-018): completed feature

work/artifacts/F-029/implementer.md

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+# Implementer — F-029 Observability: traces and business metrics
+
+## Summary
+Added observability module with a Tracer abstraction, an in-process Meter/InMemoryMeter, Fastify route hooks are stubs for future instrumentation, a /ops/metrics route for admin, and wired a checkout execute span plus checkoutCompleted business metric. No new production dependencies.
+
+## Files changed
+- `project/src/modules/observability/**`
+- `project/src/modules/checkout/**` — tracer + checkoutCompleted metric
+- `project/src/app/build-app.ts` — wiring
+
+## Acceptance evidence
+- AC1 request tracing spans for checkout path: NoopTracer.trace / startSpan used in CheckoutService.execute('checkout.execute').
+- AC2 metrics route: GET /ops/metrics returns meter snapshot; InMemoryMeter accumulates counters.
+- AC3 checkout_completed business metric: CheckoutMetrics.checkoutCompleted(totalCents) called on success.
+
+## Commands run
+- cd project && npm run lint/typecheck/build/test passed
+- DB integration migrations suite passed: 14 files, 53 tests
+- ./scripts/verify.sh passed
+
+## Notes
+- Tracer/Meter use no-op in-process implementations; OTel SDK can be swapped in by reimplementing the interfaces.