Move reranker to Serenity, add monitoring and mesh-mqtt-observer stacks

- ai: remove reranker service (moved to Serenity at 10.5.1.5:9787)
- ai: add LiteLLM reranker proxy entry for Serenity
- ai: configure OpenWebUI to use LiteLLM for reranking
- monitoring: add Grafana + Prometheus stack
- mesh-mqtt-observer: add new service
- automation, documents, meshtastic: misc updates

monitoring/prometheus.yml

@@ -0,0 +1,28 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  # ------- PlausibleDeniability (local via node-exporter) -------
+  - job_name: 'pd-node'
+    static_configs:
+      - targets: ['node-exporter:9100']
+        labels:
+          host: 'plausible-deniability'
+
+  # ------- Serenity (via Netdata Prometheus exporter) -------
+  - job_name: 'serenity-netdata'
+    metrics_path: /api/v1/allmetrics
+    params:
+      format: [prometheus]
+    static_configs:
+      - targets: ['10.5.1.5:19999']
+        labels:
+          host: 'serenity'
+
+  # ------- N.O.M.A.D. (via node-exporter) -------
+  - job_name: 'nomad-node'
+    static_configs:
+      - targets: ['10.5.1.16:9100']
+        labels:
+          host: 'nomad'