feat(ui): add Node column + filter to AuditLog grid

src/ScadaLink.CentralUI/Services/IAuditLogQueryService.cs

@@ -69,4 +69,15 @@ public interface IAuditLogQueryService
     Task<IReadOnlyList<ExecutionTreeNode>> GetExecutionTreeAsync(
         Guid executionId,
         CancellationToken ct = default);
+
+    /// <summary>
+    /// Returns the distinct, non-null <c>SourceNode</c> values present in the
+    /// central <c>AuditLog</c> table, backing the Audit Log page's Node
+    /// multi-select filter. The implementation caches the result for ~60s so
+    /// rendering the filter bar never produces more than one DB hit per minute
+    /// per circuit. The cache is process-wide — node membership changes
+    /// (failover, scaling) surface within a minute, which is acceptable for a
+    /// filter affordance.
+    /// </summary>
+    Task<IReadOnlyList<string>> GetDistinctSourceNodesAsync(CancellationToken ct = default);
 }