Adds SiteStreamManager.SubscribeSiteAlarms(IActorRef) — same broadcast-hub
wiring as Subscribe(...) but with no per-instance filter and forwarding only
AlarmStateChanged events (AttributeValueChanged dropped). Returns a subscription
id torn down via the existing Unsubscribe, symmetric with the per-instance
subscribe. Backs the Task 2 SubscribeSite gRPC server handler.
Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj