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/config/export.go

@@ -126,6 +126,10 @@ func siteToMonitor(s models.Site, alertIDToName map[int]string) Monitor {
 	m.IgnoreTLS = s.IgnoreTLS
 	m.Paused = s.Paused
 
+	if s.Regions != "" {
+		m.Regions = s.Regions
+	}
+
 	return m
 }