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.

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