feat(health): metrics-stale signal + status-transition timestamps; spec now matches heartbeat-liveness code

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/HealthMonitoringOptions.cs

@@ -19,4 +19,17 @@ public class HealthMonitoringOptions
     /// the design doc grants real sites. Default: 3x the report interval.
     /// </summary>
     public TimeSpan CentralOfflineTimeout { get; set; } = TimeSpan.FromMinutes(3);
+
+    /// <summary>
+    /// Duration an online site may go without a full health <em>report</em>
+    /// (heartbeats notwithstanding) before it is flagged
+    /// <see cref="SiteHealthState.IsMetricsStale"/>. Distinct from
+    /// <see cref="OfflineTimeout"/>, which is heartbeat-based liveness: a site
+    /// whose HealthReportSender has died keeps heartbeating and stays "online",
+    /// but its metrics freeze — this window surfaces that. Must be positive and
+    /// no shorter than <see cref="ReportInterval"/> (a window shorter than one
+    /// report interval would flag every healthy site). Default: 2 minutes
+    /// (~4 missed 30s reports).
+    /// </summary>
+    public TimeSpan MetricsStaleTimeout { get; set; } = TimeSpan.FromMinutes(2);
 }