feat(health): oldest-parked-message age on the site health report (parked-retention aging signal)

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

@@ -111,6 +111,19 @@ public interface ISiteHealthCollector
     /// <param name="count">The number of parked messages.</param>
     void SetParkedMessageCount(int count);
 
+    /// <summary>
+    /// Sets the age (seconds) of the oldest parked store-and-forward message for the
+    /// next <see cref="CollectReport"/> call; <c>null</c> when no rows are parked.
+    /// Point-in-time (not reset on collect), refreshed by the parked-count poller in
+    /// <c>HealthReportSender</c>. Default interface implementation is a no-op so
+    /// existing test fakes continue to compile without per-fake updates.
+    /// </summary>
+    /// <param name="ageSeconds">Age in seconds of the oldest parked row, or <c>null</c> if none are parked.</param>
+    void SetOldestParkedMessageAgeSeconds(double? ageSeconds)
+    {
+        // Default no-op so test fakes do not need to be updated.
+    }
+
     /// <summary>
     /// Replace the latest cumulative
     /// site-event-log write-failure count (SQLite error, disk full,