feat(phase-2): workstream 7 — Prometheus + Grafana Monitoring

- Add prom-client 15; shared registry in src/metrics/registry.ts (7 metrics)
- HTTP request counter + duration histogram via metricsMiddleware
- DB query duration histogram wrapping pg Pool.query
- Redis command duration histogram via typed instrumentRedisMethod wrapper
- agentidp_tokens_issued_total in OAuth2Service
- agentidp_agents_registered_total in AgentService
- GET /metrics unauthenticated endpoint (Prometheus text format)
- docker-compose.monitoring.yml overlay (Prometheus + Grafana)
- Grafana auto-provisioned datasource + pre-built AgentIdP dashboard
- docs/devops/operations.md monitoring section added
- 36/36 unit tests passing, 100% coverage on new metrics code
- Fix pre-existing unused import in tests/integration/agents.test.ts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

monitoring/prometheus/prometheus.yml

@@ -0,0 +1,10 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  - job_name: 'agentidp'
+    static_configs:
+      - targets: ['agentidp:3000']
+    metrics_path: /metrics
+    scheme: http