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/cmd/controller/main.go

@@ -647,6 +647,10 @@ func main() {
 	// disk execution; the controller forwards list/assign/eject/format.
 	mux.Handle("/api/disks", webServer.RequireAuth(webServer.CsrfProtect(http.HandlerFunc(webServer.ServeDiskAPI))))
 	mux.Handle("/api/disks/", webServer.RequireAuth(webServer.CsrfProtect(http.HandlerFunc(webServer.ServeDiskAPI))))
+	// Host metrics API — thin proxy to the host agent (slice 9). Read-only host-wide health +
+	// per-storage capacity for the monitoring view; the de-privileged controller can't read the
+	// host itself. GET only, so no CSRF wrapper needed.
+	mux.Handle("/api/host-metrics", webServer.RequireAuth(http.HandlerFunc(webServer.ServeHostMetricsAPI)))
 	// App export/import API routes handled by web server
 	mux.Handle("/api/export/", webServer.RequireAuth(webServer.CsrfProtect(http.HandlerFunc(webServer.ServeExportAPI))))
 	// Debug API routes handled by web server (debug-mode gating inside handler)