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/store/postgres.go

@@ -68,6 +68,7 @@ func (d *PostgresDialect) MigrationsSQL() []string {
 		"ALTER TABLE sites ADD COLUMN IF NOT EXISTS ignore_tls BOOLEAN DEFAULT FALSE",
 		"ALTER TABLE sites ADD COLUMN IF NOT EXISTS paused BOOLEAN DEFAULT FALSE",
 		"ALTER TABLE check_history ADD COLUMN IF NOT EXISTS node_id TEXT DEFAULT ''",
+		"ALTER TABLE sites ADD COLUMN IF NOT EXISTS regions TEXT DEFAULT ''",
 	}
 }