feat(sitecall-audit): add SourceNode to SiteCallOperational + SiteCall entity

src/ScadaLink.AuditLog/Site/Telemetry/CachedCallLifecycleBridge.cs

@@ -162,6 +162,9 @@ public sealed class CachedCallLifecycleBridge : ICachedCallLifecycleObserver
                 Channel: context.Channel,
                 Target: context.Target,
                 SourceSite: context.SourceSite,
+                // SourceNode: stamped by Task 14 once the bridge gets an
+                // INodeIdentityProvider; null until then.
+                SourceNode: null,
                 Status: operationalStatus,
                 RetryCount: context.RetryCount,
                 LastError: lastError,