Add Phase 5 observability and operations instrumentation

[rgomids]

Summary

• add a shared OpenTelemetry runtime for traces, metrics, propagation, and /metrics
• instrument HTTP, use cases, internal events, PostgreSQL queries, and payment gateway calls
• enrich operational logs with trace context, canonical error_type, and domain identifiers
• add local Jaeger and Prometheus support in Docker Compose and document the Phase 5 operational workflow
• update phase status, ADRs, commands, diagrams, README, AGENTS, and changelog for Phase 5

Testing

• make fmt
• make lint
• make test-unit
• make test-integration
• make test-functional
• make test