fix(health): replicate site health reports between central nodes

CentralHealthAggregator is a per-node hosted singleton, but site health
reports flow through ClusterClient which round-robins each report to one
central node only. The other node's aggregator never saw those reports
and marked sites offline at the 60s threshold — sites constantly flapped
between online and offline on the monitoring page.

On receive, the active CentralCommunicationActor now republishes a
SiteHealthReportReplica wrapper on a DistributedPubSub topic. Both
central nodes subscribe to the topic and process replicas through a
dedicated path that updates the local aggregator without re-broadcasting
(avoids fan-out loops). The aggregator's existing sequence-number
idempotency makes self-delivery a cheap no-op.

DistributedPubSubExtensionProvider is now listed in the HOCON
`akka.extensions` block so the mediator is initialised at cluster
start, eliminating a race where the first Subscribe arrived before the
extension was loaded.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -69,6 +69,9 @@ public class AkkaHostedService : IHostedService
 
         var hocon = $@"
 akka {{
+    extensions = [
+        ""Akka.Cluster.Tools.PublishSubscribe.DistributedPubSubExtensionProvider, Akka.Cluster.Tools""
+    ]
     actor {{
         provider = cluster
     }}