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/tests/test_api.py

@@ -677,6 +677,17 @@ class TestAlertmanager:
         assert data["total"] == 0
         assert data["error"] == "alertmanager_unreachable"
 
+    def test_alerts_endpoint_when_not_configured(self, test_client):
+        with patch("media_library_viewer_api.routers.monitoring._alertmanager_client") as mock_client:
+            session = MagicMock()
+            mock_client.return_value = (session, "")
+            response = test_client.get("/api/monitoring/alerts")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["total"] == 0
+        assert data["error"] == "alertmanager_not_configured"
+        session.get.assert_not_called()
+
     def test_alerts_endpoint_returns_summary(self, test_client):
         with patch("media_library_viewer_api.routers.monitoring._alertmanager_client") as mock_client:
             session = MagicMock()
@@ -700,6 +711,16 @@ class TestAlertmanager:
         assert data["by_severity"]["critical"] == 1
         assert data["alerts"][0]["name"] == "BackupJobFailed"
 
+    def test_alertmanager_status_endpoint_when_not_configured(self, test_client):
+        with patch("media_library_viewer_api.routers.monitoring._alertmanager_client") as mock_client:
+            session = MagicMock()
+            mock_client.return_value = (session, "")
+            response = test_client.get("/api/monitoring/alertmanager-status")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["up"] is False
+        session.get.assert_not_called()
+
     def test_alertmanager_status_endpoint_when_unreachable(self, test_client):
         with patch("media_library_viewer_api.routers.monitoring._alertmanager_client") as mock_client:
             session = MagicMock()