slice 9: host-health view on the monitoring page (v0.39.0)

Add agentapi HostMetrics() + a thin /api/host-metrics proxy to the agent's
new GET /host/metrics, and a 'Szerver allapota (gazdagep)' card on the
monitoring page rendering host CPU%/load/mem/CPU-temp(n/a)/uptime + per-
storage capacity bars (thin-pool fill, disk temp/wear). Polls every 8s.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/README.md

@@ -666,6 +666,14 @@ Full-page system monitor at `/monitoring`:
 
 Chart.js 4.4.7 embedded locally (works in offline environments), dark theme matching site design.
 
+#### Host (Proxmox box) Health — agent-proxied (slice 9, `internal/agentapi` + `agent_host_metrics_handler.go`)
+
+The de-privileged controller (slice 8C) sees only its own cgroup and cannot read the host. The top card of `/monitoring` ("Szerver állapota (gazdagép)") instead shows the **real Proxmox box**, proxied from the host agent's `GET /host/metrics`:
+- **Host block**: CPU% + load average, memory used/total, **CPU/chassis temperature** (or "n/a" when the hardware exposes no sensor — graceful-null), uptime.
+- **Per-storage capacity**: a used/total bar per host storage target, with thin-pool fill (a full lvmthin pool corrupts every guest on it) and disk SMART temperature/wear.
+
+Path: `GET /api/host-metrics` → `Client.HostMetrics()` (leaf-pinned, per-guest-token agentapi client) → agent `GET /host/metrics`. Host-wide and token-authed (assumption: **one customer per host** — the home-server model). It is a **live** fetch (a fresh agent collect, not the 15-minute hub snapshot), so the page polls it every **8 s** while open. When the agent is unconfigured/unreachable the card shows a "nem elérhető" banner; the controller's own metric charts are unaffected.
+
 #### Alert System (`internal/web/alerts.go`)
 
 State-based alerts displayed on all pages: