chore(communication): low-severity cleanup — eviction counting, shared ingest timeout, split app heartbeat interval, doc notes

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcServer.cs

@@ -42,7 +42,9 @@ public class SiteStreamGrpcServer : SiteStreamService.SiteStreamServiceBase
     // Ask timeout is 30 s. The ingest actor's repo
     // calls are sub-100 ms in steady state; a generous timeout absorbs a slow
     // MSSQL connection without surfacing as a gRPC failure on a healthy site.
-    private static readonly TimeSpan AuditIngestAskTimeout = TimeSpan.FromSeconds(30);
+    // Shared with CentralCommunicationActor (same assembly) so the two audit-ingest
+    // transports use one source of truth for the timeout.
+    internal static readonly TimeSpan AuditIngestAskTimeout = TimeSpan.FromSeconds(30);
     // Audit Log: site-local queue handed in by AkkaHostedService on
     // site roles so the central reconciliation puller's PullAuditEvents RPC
     // can read Pending/Forwarded rows. Null when not wired (e.g. central-only
@@ -240,7 +242,10 @@ public class SiteStreamGrpcServer : SiteStreamService.SiteStreamServiceBase
             existingEntry.Cts.Dispose();
         }
 
-        // Check max concurrent streams after duplicate removal
+        // Check max concurrent streams after duplicate removal.
+        // Deliberately check-then-act: two concurrent subscribes at the boundary can
+        // both pass, overshooting the cap by at most (concurrent subscribers - 1).
+        // Bounded and benign — not worth a lock on this path.
         if (_activeStreams.Count >= _maxConcurrentStreams)
             throw new RpcException(new GrpcStatus(StatusCode.ResourceExhausted, "Max concurrent streams reached"));
 
@@ -253,8 +258,20 @@ public class SiteStreamGrpcServer : SiteStreamService.SiteStreamServiceBase
         var entry = new StreamEntry(streamCts);
         _activeStreams[request.CorrelationId] = entry;
 
+        long dropped = 0;
+        var correlationId = request.CorrelationId;
         var channel = Channel.CreateBounded<SiteStreamEvent>(
-            new BoundedChannelOptions(1000) { FullMode = BoundedChannelFullMode.DropOldest });
+            new BoundedChannelOptions(1000) { FullMode = BoundedChannelFullMode.DropOldest },
+            _ =>
+            {
+                // Lossy-under-backpressure is the debug-view spec; make the real
+                // loss visible: first eviction + every 500th thereafter.
+                var n = Interlocked.Increment(ref dropped);
+                if (n == 1 || n % 500 == 0)
+                    _logger.LogWarning(
+                        "Debug stream {CorrelationId} backpressure: {Dropped} oldest event(s) evicted so far",
+                        correlationId, n);
+            });
 
         var actorSeq = Interlocked.Increment(ref _actorCounter);
         var relayActor = _actorSystem!.ActorOf(