feat: add incident management and maintenance windows

Maintenance windows suppress alerts during planned downtime while checks
continue running. Incidents provide informational tracking. Supports
targeting all monitors, single monitor, or group (applies to children).

New Maint tab in TUI with create/end/delete. Status page, JSON API, and
Prometheus metrics all reflect maintenance state.

internal/metrics/prometheus.go

@@ -55,6 +55,15 @@ func Handler(eng *monitor.Engine) http.HandlerFunc {
 			writeGauge(&b, "upkeep_monitor_paused", labels(s), float64(val))
 		}
 
+		writeHelp(&b, "upkeep_monitor_maintenance", "gauge", "Whether the monitor is in a maintenance window (1) or not (0).")
+		for _, s := range sites {
+			val := 0
+			if eng.GetDisplayStatus(s) == "MAINT" {
+				val = 1
+			}
+			writeGauge(&b, "upkeep_monitor_maintenance", labels(s), float64(val))
+		}
+
 		writeHelp(&b, "upkeep_monitor_cert_expiry_timestamp_seconds", "gauge", "Unix timestamp when the SSL certificate expires.")
 		for _, s := range sites {
 			if !s.HasSSL || s.CertExpiry.IsZero() {