docs: update DEPLOYMENT.md and C4 diagram for observability scaffold

Replace the stale "no monitoring infrastructure in place yet" note in
§4 with a brief description of the observability compose file and a
pointer to issue #581 for full docs.

Add a placeholder System_Boundary block for Prometheus + Loki + Grafana
to l2-containers.puml, showing the stack joins archiv-net.

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

docs/DEPLOYMENT.md

@@ -256,9 +256,9 @@ docker compose logs --tail=200 <service>
 - **Spring Actuator health**: `http://localhost:8080/actuator/health` (internal only in prod — port 8081 for Prometheus scraping)
 - **Prometheus scraping**: management port 8081, path `/actuator/prometheus`. Internal only; Caddy blocks `/actuator/*` externally.
 
-### Future observability
+### Observability stack
 
-Phase 7 of the Production v1 milestone adds Prometheus + Loki + Grafana. No monitoring infrastructure is in place yet.
+An observability stack (Prometheus + Loki + Grafana) is available via `docker-compose.observability.yml` and configuration lives under `infra/observability/`. It joins the `archiv-net` Docker network to scrape the backend's management port. Full wiring and runbook documentation is tracked in issue #581.
 
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