fix(health-monitoring): resolve HealthMonitoring-001/002 — populate S&F buffer depth, make SiteHealthState immutable

src/ScadaLink.HealthMonitoring/HealthReportSender.cs

@@ -84,6 +84,20 @@ public class HealthReportSender : BackgroundService
                         _collector.SetParkedMessageCount(parkedCount);
                     }
                     catch { /* Non-fatal — parked count will be 0 */ }
+
+                    try
+                    {
+                        // Per-category pending-message buffer depths (the documented
+                        // "store-and-forward buffer depth" triage metric). Keyed by
+                        // StoreAndForwardCategory name so the central dashboard can
+                        // render external/notification/DB-write depths separately.
+                        var depthsByCategory = await _sfStorage.GetBufferDepthByCategoryAsync();
+                        var depths = depthsByCategory.ToDictionary(
+                            kvp => kvp.Key.ToString(),
+                            kvp => kvp.Value);
+                        _collector.SetStoreAndForwardDepths(depths);
+                    }
+                    catch { /* Non-fatal — buffer depths will be empty */ }
                 }
 
                 var seq = Interlocked.Increment(ref _sequenceNumber);