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/architecture/c4/l2-containers.puml

@@ -17,6 +17,12 @@ System_Boundary(archiv, "Familienarchiv (Docker Compose)") {
     Container(mc, "Bucket / Service-Account Init", "MinIO Client (mc)", "One-shot container on startup. Idempotent: creates the archive bucket, the archiv-app service account, and attaches the readwrite policy.")
 }
 
+System_Boundary(observability, "Observability Stack (docker-compose.observability.yml / archiv-net)") {
+    Container(prometheus, "Prometheus", "prom/prometheus", "Scrapes metrics from backend management port 8081 (/actuator/prometheus). Retention and alert rules TBD — see issue #581.")
+    Container(loki, "Loki", "grafana/loki", "Log aggregation. Receives structured logs from the stack. Wiring TBD — see issue #581.")
+    Container(grafana, "Grafana", "grafana/grafana", "Dashboards and alerting UI. Data sources: Prometheus + Loki. Wiring TBD — see issue #581.")
+}
+
 Rel(user, caddy, "HTTPS", "TLS 1.2/1.3")
 Rel(caddy, frontend, "Reverse proxies non-/api requests", "HTTP / loopback:3000")
 Rel(caddy, backend, "Reverse proxies /api/*", "HTTP / loopback:8080")