feat(db): add SourceNode column to Notifications

src/ScadaLink.ConfigurationDatabase/Configurations/NotificationOutboxConfiguration.cs

@@ -47,10 +47,14 @@ public class NotificationOutboxConfiguration : IEntityTypeConfiguration<Notifica
 
         builder.Property(n => n.SourceScript).HasMaxLength(200);
 
-        // SourceNode (Audit Log #23, SourceNode-stamping): mapped in a follow-on migration
-        // (AddNotificationSourceNode). Ignored here for now so the AddAuditLogSourceNode
-        // migration only touches AuditLog. Removed in the AddNotificationSourceNode commit.
-        builder.Ignore(n => n.SourceNode);
+        // SourceNode (Audit Log #23, SourceNode-stamping): node-local identifier of the
+        // cluster member that produced the notification (e.g. "node-a", "central-a").
+        // NULL is valid for rows that pre-date this feature. ASCII — varchar(64).
+        // No index — KPIs are per-site on this table, not per-node; SourceNode is only
+        // echoed onto NotifyDeliver audit rows (#23) for cross-row correlation.
+        builder.Property(n => n.SourceNode)
+            .HasColumnType("varchar(64)")
+            .HasMaxLength(64);
 
         // OriginExecutionId (Audit Log #23): nullable uniqueidentifier carried from the
         // site so the dispatcher can echo it onto NotifyDeliver audit rows. No index —