chore(observability): externalize stack from root compose files

Manage now connects to existing Grafana/Prometheus/Alertmanager instances
and never deploys its own stack.

- docker-compose.yml / docker-compose.dev.yml: removed prometheus, loki,
  alloy, grafana, alertmanager, node-exporter services, the monitoring
  network, and observability named volumes; they now ship only backend +
  frontend. Dev frontend now joins the web network so the Vite dev proxy
  can reach the backend.
- backend: alertmanager_url default is now empty; /api/monitoring/alerts
  and /alertmanager-status return graceful "not configured" responses
  when ALERTMANAGER_URL is unset. Added not-configured tests.
- docker-compose.observability.yml: kept as the optional standalone
  example; header clarifies Manage does not deploy it.
- Removed orphaned combined monitoring/prometheus/prometheus.yml
  (standalone stack uses prometheus.standalone.yml).
- Docs (README, REQUIREMENTS decision log, monitoring-logging-design,
  observability-runbooks, context.md, MIGRATION_PLAN, frontend/README,
  CHANGELOG) updated to the connect-to-existing model.

VITE_GRAFANA_URL / VITE_PROMETHEUS_URL remain as optional frontend
deep-link overrides. .env.example still needs a manual update (safety
policy blocks assistant edits): set ALERTMANAGER_URL empty/optional and
move standalone-only vars out of the root file.

monitoring/prometheus/prometheus.yml

@@ -1,48 +0,0 @@
-global:
-  scrape_interval: 15s
-  evaluation_interval: 15s
-  external_labels:
-    monitor: manage
-
-alerting:
-  alertmanagers:
-    - static_configs:
-        - targets:
-            - alertmanager:9093
-
-rule_files:
-  - /etc/prometheus/rules/*.yml
-
-scrape_configs:
-  - job_name: prometheus
-    static_configs:
-      - targets:
-          - localhost:9090
-
-  - job_name: manage-backend
-    static_configs:
-      - targets:
-          - backend:8000
-    metrics_path: /metrics
-    scrape_interval: 15s
-
-  - job_name: node-exporter
-    static_configs:
-      - targets:
-          - node-exporter:9100
-
-  - job_name: loki
-    static_configs:
-      - targets:
-          - loki:3100
-
-  - job_name: alertmanager
-    static_configs:
-      - targets:
-          - alertmanager:9093
-
-  - job_name: node-exporter-remote
-    file_sd_configs:
-      - files:
-          - /etc/prometheus/file-sd/node_exporter_targets.json
-        refresh_interval: 30s