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

src/ScadaLink.ConfigurationDatabase/Repositories/AuditLogRepository.cs

@@ -140,6 +140,13 @@ VALUES
             query = query.Where(e => e.SourceSiteId != null && sourceSiteIds.Contains(e.SourceSiteId));
         }
 
+        // SourceNode filter mirrors SourceSiteIds: a non-empty list translates to
+        // SQL IN (…); NULL SourceNode rows are excluded when the filter is set.
+        if (filter.SourceNodes is { Count: > 0 } sourceNodes)
+        {
+            query = query.Where(e => e.SourceNode != null && sourceNodes.Contains(e.SourceNode));
+        }
+
         if (!string.IsNullOrEmpty(filter.Target))
         {
             var target = filter.Target;
@@ -761,6 +768,25 @@ VALUES
         }
     }
 
+    /// <summary>
+    /// Distinct non-null <c>SourceNode</c> values for the Audit Log page's
+    /// Node filter dropdown. EF Core translates this to
+    /// <c>SELECT DISTINCT SourceNode FROM AuditLog WHERE SourceNode IS NOT NULL ORDER BY SourceNode</c>
+    /// — a single index-less scan, but the column is bounded (one entry per
+    /// node in the cluster, currently &lt;10) and the Central UI caches the
+    /// result for ~60s, so a periodic scan is acceptable.
+    /// </summary>
+    public async Task<IReadOnlyList<string>> GetDistinctSourceNodesAsync(CancellationToken ct = default)
+    {
+        return await _context.Set<AuditEvent>()
+            .AsNoTracking()
+            .Where(e => e.SourceNode != null)
+            .Select(e => e.SourceNode!)
+            .Distinct()
+            .OrderBy(n => n)
+            .ToListAsync(ct);
+    }
+
     /// <summary>
     /// Splits a <c>STRING_AGG</c> comma-joined value into a distinct, ordered
     /// list. A null/empty aggregate (a stub node with no rows) yields an empty