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/apply.go

@@ -239,6 +239,7 @@ func monitorToSite(m Monitor, alertID, parentID int) models.Site {
 		DNSServer:      m.DNSServer,
 		IgnoreTLS:      m.IgnoreTLS,
 		Paused:         m.Paused,
+		Regions:        m.Regions,
 	}
 
 	s.ExpiryThreshold = m.ExpiryThreshold
@@ -346,6 +347,9 @@ func diffSite(existing, desired models.Site) string {
 	if existing.Paused != desired.Paused {
 		diffs = append(diffs, fmt.Sprintf("paused: %v -> %v", existing.Paused, desired.Paused))
 	}
+	if existing.Regions != desired.Regions {
+		diffs = append(diffs, fmt.Sprintf("regions: %s -> %s", existing.Regions, desired.Regions))
+	}
 	return strings.Join(diffs, ", ")
 }