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>

README.md

@@ -117,6 +117,7 @@ in [docs/installation.md](docs/installation.md#dash-agent-system-monitoring).
 
 - [docs/installation.md](docs/installation.md) — Docker, binary, agent, reverse proxy, upgrades
 - [docs/configuration.md](docs/configuration.md) — env vars, notifications, auth, boards, operations
+- [docs/monitoring.md](docs/monitoring.md) — external monitoring stack (Beszel + Prometheus/Grafana) and how Dash consumes it
 - [docs/widgets.md](docs/widgets.md) — widget development guide
 - `openapi.yaml` — API contract (generated TS client in `packages/api-client`)
 - [ROADMAP.md](ROADMAP.md) — design decisions and release history