feat: complete gRPC streaming channel — site host, docker config, docs, integration tests

Switch site host to WebApplicationBuilder with Kestrel HTTP/2 gRPC server,
add GrpcPort/keepalive config, wire SiteStreamManager as ISiteStreamSubscriber,
expose gRPC ports in docker-compose, add site seed script, update all 10
requirement docs + CLAUDE.md + README.md for the new dual-transport architecture.

src/ScadaLink.SiteRuntime/Streaming/SiteStreamManager.cs

@@ -3,6 +3,7 @@ using Akka.Actor;
 using Akka.Streams;
 using Akka.Streams.Dsl;
 using Microsoft.Extensions.Logging;
+using ScadaLink.Communication.Grpc;
 using ScadaLink.Commons.Messages.Streaming;
 
 namespace ScadaLink.SiteRuntime.Streaming;
@@ -13,10 +14,12 @@ namespace ScadaLink.SiteRuntime.Streaming;
 /// Subscribers get per-subscriber bounded buffers with drop-oldest overflow.
 ///
 /// Filterable by instance name for debug view (WP-25).
+/// Implements ISiteStreamSubscriber so the gRPC server can subscribe actors
+/// to instance events without referencing SiteRuntime directly.
 /// </summary>
-public class SiteStreamManager
+public class SiteStreamManager : ISiteStreamSubscriber
 {
-    private readonly ActorSystem _system;
+    private ActorSystem? _system;
     private readonly int _bufferSize;
     private readonly ILogger<SiteStreamManager> _logger;
     private readonly object _lock = new();
@@ -25,20 +28,21 @@ public class SiteStreamManager
     private readonly Dictionary<string, SubscriptionInfo> _subscriptions = new();
 
     public SiteStreamManager(
-        ActorSystem system,
         SiteRuntimeOptions options,
         ILogger<SiteStreamManager> logger)
     {
-        _system = system;
         _bufferSize = options.StreamBufferSize;
         _logger = logger;
     }
 
     /// <summary>
     /// Initializes the stream source. Must be called after ActorSystem is ready.
+    /// The ActorSystem is passed here rather than via the constructor so that
+    /// SiteStreamManager can be created by DI before the actor system exists.
     /// </summary>
-    public void Initialize()
+    public void Initialize(ActorSystem system)
     {
+        _system = system;
         var materializer = _system.Materializer();
 
         var source = Source.ActorRef<ISiteStreamEvent>(