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.

backend/src/media_library_viewer_api/config.py

@@ -55,7 +55,7 @@ class Settings(BaseSettings):
     # Observability
     prometheus_enabled: bool = True
     prometheus_file_sd_dir: str = "/app/backend/.cache/prometheus-file-sd"
-    alertmanager_url: str = "http://alertmanager:9093"
+    alertmanager_url: str = ""
     alertmanager_webhook_url: str = ""  # Optional receiver for alertmanager webhook notifications
 
     # Remote paths