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.Commons/Messages/Health/SiteHealthReport.cs

@@ -21,3 +21,13 @@ public record SiteHealthReport(
     IReadOnlyDictionary<string, TagQualityCounts>? DataConnectionTagQuality = null,
     int ParkedMessageCount = 0,
     IReadOnlyList<NodeStatus>? ClusterNodes = null);
+
+/// <summary>
+/// Broadcast wrapper used between central nodes to keep per-node
+/// CentralHealthAggregator state in sync. ClusterClient load-balances each
+/// incoming SiteHealthReport to one central node; that node re-publishes
+/// this wrapper on a DistributedPubSub topic so the peer node's aggregator
+/// also processes the report (idempotently — sequence numbers guard against
+/// double-counting).
+/// </summary>
+public record SiteHealthReportReplica(SiteHealthReport Report);