feat(observability): add standalone observability compose stack

Add docker-compose.observability.yml to run Grafana, Prometheus, Loki,
Alertmanager, Alloy and Node Exporter independently of Manage.
Includes a standalone Prometheus config, empty file-SD placeholder,
and runbook documentation.

monitoring/prometheus/file-sd/node_exporter_targets.json

@@ -0,0 +1 @@
+[]

monitoring/prometheus/prometheus.standalone.yml

@@ -0,0 +1,46 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+  external_labels:
+    monitor: manage-observability
+
+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: 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: grafana
+    static_configs:
+      - targets:
+          - grafana:3000
+
+  - job_name: node-exporter-remote
+    file_sd_configs:
+      - files:
+          - /etc/prometheus/file-sd/node_exporter_targets.json
+        refresh_interval: 30s