feat(infra): external monitoring stack (Beszel + Prometheus/Grafana)

Homelab monitoring design (docs/specs/2026-10-03-external-monitoring.md):
Beszel agents own the two physical hosts, prometheus-pve-exporter covers
every LXC/VM via the PVE API with no guest agents, and Grafana supplies
graphs. Dash needs zero code changes — JSON-path widgets already carry an
Authorization header and can walk Prometheus/Beszel JSON, embed widgets
iframe Grafana panels, and monitors + notification transports provide
the alerting layer for unmanaged hardware like the CM834 switch.

infra/monitoring/ ships compose.yaml, prometheus.yml, pve.yml.example
(gitignored real file), and Grafana datasource provisioning.
docs/monitoring.md documents deployment and widget recipes.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

infra/monitoring/prometheus.yml

@@ -0,0 +1,31 @@
+# Prometheus scrape config for the Dash-adjacent monitoring stack.
+# Adjust the host addresses to your LAN before starting.
+
+global:
+  scrape_interval: 30s
+  evaluation_interval: 30s
+
+scrape_configs:
+  # Proxmox cluster: node + every LXC/VM via the PVE API.
+  # No agents inside guests — pve-exporter asks the API on :8006.
+  - job_name: pve
+    metrics_path: /pve
+    params:
+      module: [default]
+      cluster: ["1"]
+      node: ["1"]
+    static_configs:
+      - targets: ["PVE_HOST:8006"]   # <- your Proxmox host
+    relabel_configs:
+      - source_labels: [__address__]
+        target_label: __param_target
+      - source_labels: [__param_target]
+        target_label: instance
+      - target_label: __address__
+        replacement: pve-exporter:9221
+
+  # Dash itself — it exposes Prometheus-format metrics already.
+  - job_name: dash
+    metrics_path: /api/metrics
+    static_configs:
+      - targets: ["DASH_HOST:3000"]   # <- wherever Dash runs (CasaOS box, etc.)