v0.2.0 — Quantitative observability via Prometheus and Grafana

Adds native Prometheus metrics across all seven services (Micrometer for the four Spring Boot services, promhttp for the two Go services, prometheus_client for notification-service) plus RabbitMQ's built-in metrics plugin, scraped by a provisioned Prometheus and visualized in seven pre-built Grafana dashboards (per-language infra dashboards plus a business-metrics dashboard covering orders, payments, inventory reservations, and notifications). Complements the qualitative tracing/logging observability introduced earlier (ADR-0024) without adding a tracing backend, Alertmanager, or a log aggregator — see ADR-0036 for the full rationale and scope boundaries. Current service status in the README.