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/ServiceCollectionExtensions.cs

@@ -1,9 +1,12 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ScadaLink.Communication.Grpc;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.SiteRuntime.Persistence;
 using ScadaLink.SiteRuntime.Repositories;
 using ScadaLink.SiteRuntime.Scripts;
+using ScadaLink.SiteRuntime.Streaming;
 
 namespace ScadaLink.SiteRuntime;
 
@@ -40,6 +43,17 @@ public static class ServiceCollectionExtensions
         // WP-17: Shared script library
         services.AddSingleton<SharedScriptLibrary>();
 
+        // WP-23: Site stream manager — registered as singleton and exposed as ISiteStreamSubscriber
+        // so the gRPC server can subscribe relay actors to instance events.
+        // ActorSystem is injected later via Initialize() after AkkaHostedService starts.
+        services.AddSingleton(sp =>
+        {
+            var options = sp.GetRequiredService<IOptions<SiteRuntimeOptions>>().Value;
+            var logger = sp.GetRequiredService<ILogger<SiteStreamManager>>();
+            return new SiteStreamManager(options, logger);
+        });
+        services.AddSingleton<ISiteStreamSubscriber>(sp => sp.GetRequiredService<SiteStreamManager>());
+
         // Site-local repository implementations backed by SQLite
         services.AddScoped<IExternalSystemRepository, SiteExternalSystemRepository>();
         services.AddScoped<INotificationRepository, SiteNotificationRepository>();