feat(proxmox): add node-level stats endpoint with streaming support

Add new `/proxmox/stats/{node}` API endpoint for retrieving Proxmox node
statistics in JSON format. The endpoint returns kernel version, CPU
usage/model, memory usage, rootfs usage, uptime, and load averages.

The existing `/proxmox/stats/{node}/{vmid}` endpoint has been corrected `VMStats` to return`text/plain` instead of `application/json`.

Both endpoints support WebSocket streaming for real-time stats updates
with a 1-second poll interval.

internal/proxmox/node_stats.go

@@ -0,0 +1,143 @@
+package proxmox
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+	"time"
+)
+
+type NodeStats struct {
+	KernelVersion string `json:"kernel_version"`
+	PVEVersion    string `json:"pve_version"`
+	CPUUsage      string `json:"cpu_usage"`
+	CPUModel      string `json:"cpu_model"`
+	MemUsage      string `json:"mem_usage"`
+	MemTotal      string `json:"mem_total"`
+	MemPct        string `json:"mem_pct"`
+	RootFSUsage   string `json:"rootfs_usage"`
+	RootFSTotal   string `json:"rootfs_total"`
+	RootFSPct     string `json:"rootfs_pct"`
+	Uptime        string `json:"uptime"`
+	LoadAvg1m     string `json:"load_avg_1m"`
+	LoadAvg5m     string `json:"load_avg_5m"`
+	LoadAvg15m    string `json:"load_avg_15m"`
+}
+
+// NodeStats streams node stats, like docker stats.
+func (n *Node) NodeStats(ctx context.Context, stream bool) (io.ReadCloser, error) {
+	if !stream {
+		var buf bytes.Buffer
+		if err := n.writeNodeStatsLine(ctx, &buf); err != nil {
+			return nil, err
+		}
+		return io.NopCloser(&buf), nil
+	}
+
+	pr, pw := io.Pipe()
+
+	go func() {
+		writeSample := func() error {
+			return n.writeNodeStatsLine(ctx, pw)
+		}
+
+		// Match `watch` behavior: write immediately, then on each tick.
+		if err := writeSample(); err != nil {
+			_ = pw.CloseWithError(err)
+			return
+		}
+
+		ticker := time.NewTicker(NodeStatsPollInterval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				_ = pw.CloseWithError(ctx.Err())
+				return
+			case <-ticker.C:
+				if err := writeSample(); err != nil {
+					_ = pw.CloseWithError(err)
+					return
+				}
+			}
+		}
+	}()
+
+	return pr, nil
+}
+
+func (n *Node) writeNodeStatsLine(ctx context.Context, w io.Writer) error {
+	// Fetch node status for CPU and memory metrics.
+	node, err := n.client.Node(ctx, n.name)
+	if err != nil {
+		return err
+	}
+
+	cpu := fmt.Sprintf("%.1f%%", node.CPU*100)
+
+	memUsage := formatIECBytes(node.Memory.Used)
+	memTotal := formatIECBytes(node.Memory.Total)
+	memPct := "0.00%"
+	if node.Memory.Total > 0 {
+		memPct = fmt.Sprintf("%.2f%%", float64(node.Memory.Used)/float64(node.Memory.Total)*100)
+	}
+
+	rootFSUsage := formatIECBytes(node.RootFS.Used)
+	rootFSTotal := formatIECBytes(node.RootFS.Total)
+	rootFSPct := "0.00%"
+	if node.RootFS.Total > 0 {
+		rootFSPct = fmt.Sprintf("%.2f%%", float64(node.RootFS.Used)/float64(node.RootFS.Total)*100)
+	}
+
+	uptime := formatDuration(node.Uptime)
+
+	if len(node.LoadAvg) != 3 {
+		return fmt.Errorf("unexpected load average length: %d, expected 3 (1m, 5m, 15m)", len(node.LoadAvg))
+	}
+
+	// Linux 6.17.4-1-pve #1 SMP PREEMPT_DYNAMIC PMX 6.17.4-1 (2025-12-03T15:42Z)
+	// => 6.17.4-1-pve #1 SMP PREEMPT_DYNAMIC PMX 6.17.4-1 (2025-12-03T15:42Z)
+	kversion, _ := strings.CutPrefix(node.Kversion, "Linux ")
+	// => 6.17.4-1-pve
+	kversion, _, _ = strings.Cut(kversion, " ")
+
+	nodeStats := NodeStats{
+		KernelVersion: kversion,
+		PVEVersion:    node.PVEVersion,
+		CPUUsage:      cpu,
+		CPUModel:      node.CPUInfo.Model,
+		MemUsage:      memUsage,
+		MemTotal:      memTotal,
+		MemPct:        memPct,
+		RootFSUsage:   rootFSUsage,
+		RootFSTotal:   rootFSTotal,
+		RootFSPct:     rootFSPct,
+		Uptime:        uptime,
+		LoadAvg1m:     node.LoadAvg[0],
+		LoadAvg5m:     node.LoadAvg[1],
+		LoadAvg15m:    node.LoadAvg[2],
+	}
+
+	err = json.NewEncoder(w).Encode(nodeStats)
+	return err
+}
+
+// formatDuration formats uptime in seconds to a human-readable string.
+func formatDuration(seconds uint64) string {
+	if seconds < 60 {
+		return fmt.Sprintf("%ds", seconds)
+	}
+	days := seconds / 86400
+	hours := (seconds % 86400) / 3600
+	mins := (seconds % 3600) / 60
+	if days > 0 {
+		return fmt.Sprintf("%dd%dh%dm", days, hours, mins)
+	}
+	if hours > 0 {
+		return fmt.Sprintf("%dh%dm", hours, mins)
+	}
+	return fmt.Sprintf("%dm", mins)
+}