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/types.go

@@ -30,5 +30,6 @@ type Monitor struct {
 	DNSServer       string    `yaml:"dns_server,omitempty"`
 	IgnoreTLS       bool      `yaml:"ignore_tls,omitempty"`
 	Paused          bool      `yaml:"paused,omitempty"`
+	Regions         string    `yaml:"regions,omitempty"`
 	Monitors        []Monitor `yaml:"monitors,omitempty"`
 }