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/grafana/datasources.yaml

@@ -0,0 +1,10 @@
+# Auto-provisions the Prometheus datasource so Grafana works on first boot.
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: true