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.

docker/site-b-node-a/appsettings.Site.json

@@ -4,7 +4,8 @@
       "Role": "Site",
       "NodeHostname": "scadalink-site-b-a",
       "SiteId": "site-b",
-      "RemotingPort": 8082
+      "RemotingPort": 8082,
+      "GrpcPort": 8083
     },
     "Cluster": {
       "SeedNodes": [