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.

CHANGELOG.md

@@ -4,6 +4,25 @@ All notable changes to Manage. Breaking changes are marked with **BREAKING**.
 
 ## [Unreleased]
 
+### Changed — Observability is now external only
+
+- **Removed** all observability services from `docker-compose.yml` and
+  `docker-compose.dev.yml`. They now deploy **only** the backend and frontend.
+  The `monitoring` network and the `prometheus`/`loki`/`alloy`/`grafana`/
+  `alertmanager`/`node-exporter` services and their named volumes were deleted,
+  and the `GRAFANA_APP_HOST` Traefik rule was removed.
+- Manage now connects to **existing** Grafana/Prometheus/Alertmanager instances
+  and never ships its own stack. The previous in-compose stack is preserved as
+  an optional, deploy-it-yourself example in `docker-compose.observability.yml`
+  (config under `monitoring/`, documented in `docs/observability-runbooks.md`).
+- The backend `alertmanager_url` default is now empty. The
+  `/api/monitoring/alerts` and `/alertmanager-status` endpoints return a
+  graceful "not configured" response when `ALERTMANAGER_URL` is unset.
+- Removed the now-orphaned combined `monitoring/prometheus/prometheus.yml`; the
+  standalone stack uses `monitoring/prometheus/prometheus.standalone.yml`.
+- `VITE_GRAFANA_URL`/`VITE_PROMETHEUS_URL` remain as optional frontend deep-link
+  overrides. `ALERTMANAGER_URL`/`ALERTMANAGER_WEBHOOK_URL` are optional.
+
 ### Added — Service registry
 
 - Runtime **service registry** persisted in the backend SQLite database. External