feat(sitestream): add additive SubscribeSite alarm-only gRPC stream + server handler (plan #10 T2)

Adds an additive, site-wide, alarm-only gRPC stream backing the aggregated
Alarm Summary. Proto: new SiteStreamRequest { correlation_id } message + rpc
SubscribeSite(SiteStreamRequest) returns (stream SiteStreamEvent) on
SiteStreamService — purely additive, no field renumbering. Regenerated the
checked-in SiteStreamGrpc/*.cs.

Server: SubscribeInstance and the new SubscribeSite now delegate to a shared
RunSubscriptionStreamAsync helper (readiness/shutdown guards, correlation-id
validation, duplicate replacement, concurrency cap, bounded DropOldest channel,
relay actor, SiteConnectionOpened/Closed telemetry, guaranteed cleanup). The
only variation is the subscribe delegate: SubscribeSite calls
ISiteStreamSubscriber.SubscribeSiteAlarms (no per-instance filter). Added
SubscribeSiteAlarms to the ISiteStreamSubscriber contract (SiteStreamManager
already implements it from T1). StreamRelayActor reused unchanged — it already
drops IsConfiguredPlaceholder rows and maps the enriched AlarmStateUpdate.

Tests: SubscribeSite subscribes site alarms + removes on cancel, rejects unsafe
correlation ids, and relays a domain AlarmStateChanged as a proto
AlarmStateUpdate on the stream.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.Communication/Protos/sitestream.proto

@@ -7,6 +7,10 @@ import "google/protobuf/wrappers.proto";   // Int32Value
 
 service SiteStreamService {
   rpc SubscribeInstance(InstanceStreamRequest) returns (stream SiteStreamEvent);
+  // Site-wide, alarm-only live stream (aggregated Alarm Summary): every
+  // AlarmStateChanged for ALL instances on the site, no per-instance filter.
+  // Attribute updates are never carried on this stream.
+  rpc SubscribeSite(SiteStreamRequest) returns (stream SiteStreamEvent);
   rpc IngestAuditEvents(AuditEventBatch) returns (IngestAck);
   rpc IngestCachedTelemetry(CachedTelemetryBatch) returns (IngestAck);
   rpc PullAuditEvents(PullAuditEventsRequest) returns (PullAuditEventsResponse);
@@ -18,6 +22,13 @@ message InstanceStreamRequest {
   string instance_unique_name = 2;
 }
 
+// Request for the site-wide, alarm-only SubscribeSite stream. Unlike
+// InstanceStreamRequest there is NO instance filter — the stream carries alarm
+// transitions for every instance on the site.
+message SiteStreamRequest {
+  string correlation_id = 1;
+}
+
 message SiteStreamEvent {
   string correlation_id = 1;
   oneof event {