feat(cluster): add region affinity, Nodes TUI tab, and probe metrics

Phase 3 of distributed probing:
- Add regions column to sites table for per-monitor probe affinity
- Region-filtered probe assignments (empty regions = all probes)
- New Nodes TUI tab showing connected probes with status/region/last-seen
- Regions input field in site form for configuring probe affinity
- Config-as-code support for regions (export/import/diff)
- Prometheus upkeep_probe_up metric with per-node labels
- Reindex TUI tabs: Sites, Alerts, Logs, Nodes, Users

internal/tui/tab_nodes.go

@@ -0,0 +1,96 @@
+package tui
+
+import (
+	"fmt"
+	"go-upkeep/internal/models"
+	"strings"
+	"time"
+)
+
+func (m Model) viewNodesTab() string {
+	if len(m.nodes) == 0 {
+		return "\n  No probe nodes connected."
+	}
+
+	colWidths := []int{0, 12, 20, 10, 8}
+
+	return m.renderTable(
+		[]string{"NAME", "REGION", "LAST SEEN", "VERSION", "STATUS"},
+		len(m.nodes),
+		func(start, end int) [][]string {
+			var rows [][]string
+			for i := start; i < end; i++ {
+				node := m.nodes[i]
+				name := limitStr(node.Name, 20)
+				if name == "" {
+					name = node.ID
+				}
+				region := node.Region
+				if region == "" {
+					region = subtleStyle.Render("—")
+				}
+				lastSeen := fmtNodeLastSeen(node.LastSeen)
+				version := node.Version
+				if version == "" {
+					version = subtleStyle.Render("—")
+				}
+				status := fmtNodeStatus(node.LastSeen)
+				rows = append(rows, []string{name, region, lastSeen, version, status})
+			}
+			return rows
+		},
+		colWidths,
+		nil,
+	)
+}
+
+func fmtNodeStatus(lastSeen time.Time) string {
+	if lastSeen.IsZero() {
+		return subtleStyle.Render("UNKNOWN")
+	}
+	ago := time.Since(lastSeen)
+	if ago < 60*time.Second {
+		return specialStyle.Render("ONLINE")
+	}
+	if ago < 5*time.Minute {
+		return warnStyle.Render("STALE")
+	}
+	return dangerStyle.Render("OFFLINE")
+}
+
+func fmtNodeLastSeen(t time.Time) string {
+	if t.IsZero() {
+		return subtleStyle.Render("never")
+	}
+	ago := time.Since(t)
+	if ago < time.Minute {
+		return fmt.Sprintf("%ds ago", int(ago.Seconds()))
+	}
+	if ago < time.Hour {
+		return fmt.Sprintf("%dm ago", int(ago.Minutes()))
+	}
+	return fmt.Sprintf("%dh ago", int(ago.Hours()))
+}
+
+func fmtProbeRegions(site models.Site, probeResults map[string]probeStatus) string {
+	if len(probeResults) == 0 {
+		return subtleStyle.Render("—")
+	}
+	var parts []string
+	for region, status := range probeResults {
+		short := region
+		if len(short) > 6 {
+			short = short[:6]
+		}
+		if status.isUp {
+			parts = append(parts, specialStyle.Render(short+":UP"))
+		} else {
+			parts = append(parts, dangerStyle.Render(short+":DN"))
+		}
+	}
+	return strings.Join(parts, " ")
+}
+
+type probeStatus struct {
+	isUp bool
+}