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.

CLAUDE.md

@@ -11,8 +11,7 @@ This project contains design documentation for a distributed SCADA system built
 - `AkkaDotNet/` — Akka.NET reference documentation and best practices notes.
 - `test_infra.md` — Master test infrastructure doc (OPC UA, LDAP, MS SQL).
 - `infra/` — Docker Compose and config files for local test services.
-
-There is no source code in this project — only design documentation in markdown.
+- `docker/` — Docker infrastructure for the 8-node cluster topology (2 central + 3 sites). See [`docker/README.md`](docker/README.md) for cluster setup, port allocation, and management commands.
 
 ## Document Conventions
 
@@ -79,6 +78,7 @@ There is no source code in this project — only design documentation in markdow
 - Tag path resolution retried periodically for devices still booting.
 - Static attribute writes persisted to local SQLite (survive restart/failover, reset on redeployment).
 - All timestamps are UTC throughout the system.
+- Site addressing is database-driven: NodeAAddress and NodeBAddress stored in the Sites table, cached in CentralCommunicationActor, refreshed periodically (60s) and on admin changes. Heartbeats serve health monitoring only.
 
 ### External Integrations
 - External System Gateway: HTTP/REST only, JSON serialization, API key + Basic Auth.