Fix nil pointer in NewSearchServer - use serverOptions.Backend

The backend may be created inside the function and assigned to
serverOptions.Backend, not opts.Backend. Using opts.Backend caused
a nil pointer dereference in searchSupport.buildIndexes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/storage/unified/sql/server.go

@@ -159,7 +159,7 @@ func NewResourceServer(opts ServerOptions) (resource.ResourceServer, resource.Se
 		}
 	}
 
-	search, err := resource.NewSearchServer(opts.SearchOptions, opts.Backend, opts.AccessClient, nil, opts.IndexMetrics, opts.OwnsIndexFn)
+	search, err := resource.NewSearchServer(opts.SearchOptions, serverOptions.Backend, opts.AccessClient, nil, opts.IndexMetrics, opts.OwnsIndexFn)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to initialize search: %w", err)
 	}