fix(health): route site heartbeats into the aggregator

CentralCommunicationActor.HandleHeartbeat was forwarding each incoming
HeartbeatMessage to Context.Parent, which resolves to the /user
guardian — a non-actor. Every site heartbeat went straight to dead
letters (~1026 per central node per 30 minutes at the default ~2s
interval across three sites).

The aggregator now exposes MarkHeartbeat(siteId, receivedAt) which
bumps LastReportReceivedAt on already-known sites (and clears IsOnline
if it had flipped) without touching LatestReport. Heartbeats from
unregistered sites are dropped — first registration still happens on
the first full report. CentralCommunicationActor calls this in place
of the no-op Tell.

The result: heartbeats now serve their stated health-monitoring
purpose (per CLAUDE.md) by keeping a site marked online between the
30s full reports if a single report is briefly delayed, and the dead
letter noise disappears entirely.

src/ScadaLink.Communication/Actors/CentralCommunicationActor.cs

@@ -100,8 +100,8 @@ public class CentralCommunicationActor : ReceiveActor
 
     private void HandleHeartbeat(HeartbeatMessage heartbeat)
     {
-        // Forward heartbeat to parent for any interested central actors
-        Context.Parent.Tell(heartbeat);
+        var aggregator = _serviceProvider.GetService<ICentralHealthAggregator>();
+        aggregator?.MarkHeartbeat(heartbeat.SiteId, heartbeat.Timestamp);
     }
 
     /// <summary>

src/ScadaLink.HealthMonitoring/CentralHealthAggregator.cs

@@ -76,6 +76,26 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
             });
     }
 
+    /// <summary>
+    /// Bumps the last-seen timestamp for a site already known via a prior
+    /// SiteHealthReport. Heartbeats from sites we have not yet received a
+    /// full report from are ignored — registration only happens on report.
+    /// </summary>
+    public void MarkHeartbeat(string siteId, DateTimeOffset receivedAt)
+    {
+        if (!_siteStates.TryGetValue(siteId, out var state))
+            return;
+
+        if (receivedAt > state.LastReportReceivedAt)
+            state.LastReportReceivedAt = receivedAt;
+
+        if (!state.IsOnline)
+        {
+            state.IsOnline = true;
+            _logger.LogInformation("Site {SiteId} is back online (heartbeat)", siteId);
+        }
+    }
+
     /// <summary>
     /// Get the current health state for all known sites.
     /// </summary>

src/ScadaLink.HealthMonitoring/ICentralHealthAggregator.cs

@@ -9,6 +9,15 @@ namespace ScadaLink.HealthMonitoring;
 public interface ICentralHealthAggregator
 {
     void ProcessReport(SiteHealthReport report);
+
+    /// <summary>
+    /// Bumps the last-seen timestamp for a site already known via a prior
+    /// SiteHealthReport. Used to keep a site marked online between full
+    /// 30s reports when ~2s heartbeats are arriving — protects against the
+    /// 60s offline threshold firing on a transiently delayed report.
+    /// </summary>
+    void MarkHeartbeat(string siteId, DateTimeOffset receivedAt);
+
     IReadOnlyDictionary<string, SiteHealthState> GetAllSiteStates();
     SiteHealthState? GetSiteState(string siteId);
 }

tests/ScadaLink.Communication.Tests/CentralCommunicationActorTests.cs

@@ -11,6 +11,7 @@ using ScadaLink.Commons.Messages.Deployment;
 using ScadaLink.Commons.Messages.DebugView;
 using ScadaLink.Commons.Messages.Health;
 using ScadaLink.Communication.Actors;
+using ScadaLink.HealthMonitoring;
 using Akka.TestKit;
 
 namespace ScadaLink.Communication.Tests;
@@ -140,25 +141,27 @@ public class CentralCommunicationActorTests : TestKit
     }
 
     [Fact]
-    public void Heartbeat_ForwardedToParent()
+    public void Heartbeat_BumpsAggregatorTimestamp()
     {
         var mockRepo = Substitute.For<ISiteRepository>();
         mockRepo.GetAllSitesAsync(Arg.Any<CancellationToken>())
             .Returns(new List<Site>());
 
+        var aggregator = Substitute.For<ICentralHealthAggregator>();
+
         var services = new ServiceCollection();
         services.AddScoped(_ => mockRepo);
+        services.AddSingleton(aggregator);
         var sp = services.BuildServiceProvider();
 
         var siteClientFactory = Substitute.For<ISiteClientFactory>();
-        var parentProbe = CreateTestProbe();
-        var centralActor = parentProbe.ChildActorOf(
+        var centralActor = Sys.ActorOf(
             Props.Create(() => new CentralCommunicationActor(sp, siteClientFactory)));
 
-        var heartbeat = new HeartbeatMessage("site1", "host1", true, DateTimeOffset.UtcNow);
-        centralActor.Tell(heartbeat);
+        var timestamp = DateTimeOffset.UtcNow;
+        centralActor.Tell(new HeartbeatMessage("site1", "host1", true, timestamp));
 
-        parentProbe.ExpectMsg<HeartbeatMessage>(msg => msg.SiteId == "site1");
+        AwaitAssert(() => aggregator.Received(1).MarkHeartbeat("site1", timestamp));
     }
 
     [Fact]

tests/ScadaLink.Communication.Tests/ScadaLink.Communication.Tests.csproj

@@ -25,6 +25,7 @@
   <ItemGroup>
     <ProjectReference Include="../../src/ScadaLink.Communication/ScadaLink.Communication.csproj" />
     <ProjectReference Include="../../src/ScadaLink.Commons/ScadaLink.Commons.csproj" />
+    <ProjectReference Include="../../src/ScadaLink.HealthMonitoring/ScadaLink.HealthMonitoring.csproj" />
   </ItemGroup>
 
 </Project>