feat: replace site registration with database-driven site addressing

Central now resolves site Akka remoting addresses from the Sites DB table
(NodeAAddress/NodeBAddress) instead of relying on runtime RegisterSite
messages. Eliminates the race condition where sites starting before central
had their registration dead-lettered. Addresses are cached in
CentralCommunicationActor with 60s periodic refresh and on-demand refresh
when sites are added/edited/deleted via UI or CLI.

src/ScadaLink.Communication/Actors/CentralCommunicationActor.cs

@@ -1,5 +1,7 @@
 using Akka.Actor;
 using Akka.Event;
+using Microsoft.Extensions.DependencyInjection;
+using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Messages.Communication;
 using ScadaLink.Commons.Messages.Health;
 
@@ -7,7 +9,7 @@ namespace ScadaLink.Communication.Actors;
 
 /// <summary>
 /// Central-side actor that routes messages from central to site clusters via Akka remoting.
-/// Maintains a registry of known site actor paths (learned from heartbeats/connection events).
+/// Resolves site addresses from the database on a periodic refresh cycle.
 ///
 /// WP-4: All 8 message patterns routed through this actor.
 /// WP-5: Ask timeout on connection drop (no central buffering). Debug streams killed on interruption.
@@ -15,12 +17,14 @@ namespace ScadaLink.Communication.Actors;
 public class CentralCommunicationActor : ReceiveActor
 {
     private readonly ILoggingAdapter _log = Context.GetLogger();
+    private readonly IServiceProvider _serviceProvider;
 
     /// <summary>
-    /// Maps SiteId → remote SiteCommunicationActor selection.
-    /// Updated when heartbeats arrive or connection state changes.
+    /// Cached site address entries loaded from the database.
+    /// Maps SiteIdentifier → (NodeA selection, NodeB selection).
+    /// Refreshed periodically via RefreshSiteAddresses.
     /// </summary>
-    private readonly Dictionary<string, ActorSelection> _siteSelections = new();
+    private Dictionary<string, (ActorSelection? NodeA, ActorSelection? NodeB)> _siteAddressCache = new();
 
     /// <summary>
     /// Tracks active debug view subscriptions: correlationId → (siteId, subscriber).
@@ -34,31 +38,30 @@ public class CentralCommunicationActor : ReceiveActor
     /// </summary>
     private readonly Dictionary<string, string> _inProgressDeployments = new();
 
-    public CentralCommunicationActor()
+    private ICancelable? _refreshSchedule;
+
+    public CentralCommunicationActor(IServiceProvider serviceProvider)
     {
+        _serviceProvider = serviceProvider;
+
+        // Site address cache loaded from database
+        Receive<SiteAddressCacheLoaded>(HandleSiteAddressCacheLoaded);
+
+        // Periodic refresh trigger
+        Receive<RefreshSiteAddresses>(_ => LoadSiteAddressesFromDb());
+
         // Site registration via heartbeats
         Receive<HeartbeatMessage>(HandleHeartbeat);
 
         // Connection state changes
         Receive<ConnectionStateChanged>(HandleConnectionStateChanged);
 
-        // Site registration command (manual or from discovery)
-        Receive<RegisterSite>(HandleRegisterSite);
-
         // Route enveloped messages to sites
         Receive<SiteEnvelope>(HandleSiteEnvelope);
     }
 
     private void HandleHeartbeat(HeartbeatMessage heartbeat)
     {
-        // Heartbeats arrive from sites — forward to any interested central actors
-        // The sender's path tells us the site's communication actor address
-        if (!_siteSelections.ContainsKey(heartbeat.SiteId))
-        {
-            var senderPath = Sender.Path.ToString();
-            _log.Info("Learned site {0} from heartbeat at path {1}", heartbeat.SiteId, senderPath);
-        }
-
         // Forward heartbeat to parent/subscribers (central health monitoring)
         Context.Parent.Tell(heartbeat);
     }
@@ -94,7 +97,7 @@ public class CentralCommunicationActor : ReceiveActor
                 _inProgressDeployments.Remove(deploymentId);
             }
 
-            _siteSelections.Remove(msg.SiteId);
+            // Note: Do NOT remove from _siteAddressCache — addresses are persistent in the database
         }
         else
         {
@@ -102,30 +105,71 @@ public class CentralCommunicationActor : ReceiveActor
         }
     }
 
-    private void HandleRegisterSite(RegisterSite msg)
-    {
-        var selection = Context.ActorSelection(msg.RemoteActorPath);
-        _siteSelections[msg.SiteId] = selection;
-        _log.Info("Registered site {0} at path {1}", msg.SiteId, msg.RemoteActorPath);
-    }
-
     private void HandleSiteEnvelope(SiteEnvelope envelope)
     {
-        if (!_siteSelections.TryGetValue(envelope.SiteId, out var siteSelection))
+        if (!_siteAddressCache.TryGetValue(envelope.SiteId, out var entry))
         {
-            _log.Warning("No known path for site {0}, cannot route message {1}",
+            _log.Warning("No known address for site {0}, cannot route message {1}",
                 envelope.SiteId, envelope.Message.GetType().Name);
 
             // The Ask will timeout on the caller side — no central buffering (WP-5)
             return;
         }
 
+        // Prefer NodeA, fall back to NodeB
+        var selection = entry.NodeA ?? entry.NodeB;
+        if (selection == null)
+        {
+            _log.Warning("Site {0} has no configured node addresses, cannot route message {1}",
+                envelope.SiteId, envelope.Message.GetType().Name);
+            return;
+        }
+
         // Track debug subscriptions for cleanup on disconnect
         TrackMessageForCleanup(envelope);
 
         // Forward the inner message to the site, preserving the original sender
         // so the site can reply directly to the caller (completing the Ask pattern)
-        siteSelection.Tell(envelope.Message, Sender);
+        selection.Tell(envelope.Message, Sender);
+    }
+
+    private void LoadSiteAddressesFromDb()
+    {
+        var self = Self;
+        Task.Run(async () =>
+        {
+            using var scope = _serviceProvider.CreateScope();
+            var repo = scope.ServiceProvider.GetRequiredService<ISiteRepository>();
+            var sites = await repo.GetAllSitesAsync();
+
+            var cache = new Dictionary<string, (string? NodeAAddress, string? NodeBAddress)>();
+            foreach (var site in sites)
+            {
+                if (string.IsNullOrWhiteSpace(site.NodeAAddress) && string.IsNullOrWhiteSpace(site.NodeBAddress))
+                    continue;
+
+                cache[site.SiteIdentifier] = (site.NodeAAddress, site.NodeBAddress);
+            }
+
+            return new SiteAddressCacheLoaded(cache);
+        }).PipeTo(self);
+    }
+
+    private void HandleSiteAddressCacheLoaded(SiteAddressCacheLoaded msg)
+    {
+        var newCache = new Dictionary<string, (ActorSelection? NodeA, ActorSelection? NodeB)>();
+        foreach (var (siteId, (nodeAAddr, nodeBAddr)) in msg.Addresses)
+        {
+            var nodeA = !string.IsNullOrWhiteSpace(nodeAAddr)
+                ? Context.ActorSelection(nodeAAddr)
+                : null;
+            var nodeB = !string.IsNullOrWhiteSpace(nodeBAddr)
+                ? Context.ActorSelection(nodeBAddr)
+                : null;
+            newCache[siteId] = (nodeA, nodeB);
+        }
+        _siteAddressCache = newCache;
+        _log.Info("Site address cache refreshed with {0} site(s)", _siteAddressCache.Count);
     }
 
     private void TrackMessageForCleanup(SiteEnvelope envelope)
@@ -149,11 +193,20 @@ public class CentralCommunicationActor : ReceiveActor
     protected override void PreStart()
     {
         _log.Info("CentralCommunicationActor started");
+
+        // Schedule periodic refresh of site addresses from the database
+        _refreshSchedule = Context.System.Scheduler.ScheduleTellRepeatedlyCancelable(
+            TimeSpan.Zero,
+            TimeSpan.FromSeconds(60),
+            Self,
+            new RefreshSiteAddresses(),
+            ActorRefs.NoSender);
     }
 
     protected override void PostStop()
     {
         _log.Info("CentralCommunicationActor stopped. In-progress deployments treated as failed (WP-5).");
+        _refreshSchedule?.Cancel();
         // On central failover, all in-progress deployments are failed
         _inProgressDeployments.Clear();
         _debugSubscriptions.Clear();
@@ -161,9 +214,15 @@ public class CentralCommunicationActor : ReceiveActor
 }
 
 /// <summary>
-/// Command to register a site's remote communication actor path.
+/// Command to trigger a refresh of site addresses from the database.
 /// </summary>
-public record RegisterSite(string SiteId, string RemoteActorPath);
+public record RefreshSiteAddresses;
+
+/// <summary>
+/// Internal message carrying the loaded site address data from the database.
+/// ActorSelection creation happens on the actor thread in HandleSiteAddressCacheLoaded.
+/// </summary>
+internal record SiteAddressCacheLoaded(Dictionary<string, (string? NodeAAddress, string? NodeBAddress)> Addresses);
 
 /// <summary>
 /// Notification sent to debug view subscribers when the stream is terminated

src/ScadaLink.Communication/CommunicationService.cs

@@ -9,6 +9,7 @@ using ScadaLink.Commons.Messages.InboundApi;
 using ScadaLink.Commons.Messages.Integration;
 using ScadaLink.Commons.Messages.Lifecycle;
 using ScadaLink.Commons.Messages.RemoteQuery;
+using ScadaLink.Communication.Actors;
 
 namespace ScadaLink.Communication;
 
@@ -39,6 +40,14 @@ public class CommunicationService
         _centralCommunicationActor = centralCommunicationActor;
     }
 
+    /// <summary>
+    /// Triggers an immediate refresh of the site address cache from the database.
+    /// </summary>
+    public void RefreshSiteAddresses()
+    {
+        GetActor().Tell(new RefreshSiteAddresses());
+    }
+
     private IActorRef GetActor()
     {
         return _centralCommunicationActor