feat(audit): stamp SourceNode at CentralAuditWriter + persist via AuditLogRepository

CentralAuditWriter injects INodeIdentityProvider and stamps the event before
handing to the repository. AuditLogRepository.InsertIfNotExistsAsync now
includes SourceNode in the INSERT column list. Caller-provided value wins
(supports any future direct-write callsite that already has its own node id).

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -183,7 +183,14 @@ public static class ServiceCollectionExtensions
             sp,
             sp.GetRequiredService<ILogger<CentralAuditWriter>>(),
             sp.GetRequiredService<IAuditPayloadFilter>(),
-            sp.GetRequiredService<ICentralAuditWriteFailureCounter>()));
+            sp.GetRequiredService<ICentralAuditWriteFailureCounter>(),
+            // SourceNode-stamping (Task 12): wire the local node identity so
+            // central-origin rows (Notification Outbox dispatch, Inbound API)
+            // carry the writing node's identifier when the caller hasn't
+            // already supplied one. GetRequiredService — the production
+            // composition root in SiteServiceRegistration registers the
+            // provider as a singleton on both site and central paths.
+            sp.GetRequiredService<INodeIdentityProvider>()));
 
         return services;
     }