fix: KB Accelerator tree builder, approve all, canvas delete button

- Fix _build_troubleshooting_tree() to handle deep nesting, warning nodes,
  and DAG deduplication (placed set prevents duplicate IDs)
- Fix step_sync VARCHAR(255) overflow on publish by truncating title
- Add "Approve All" button to KB review screen
- Add delete button (hover-reveal) to flow canvas nodes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/tree-editor/TreeEditorLayout.tsx

@@ -19,6 +19,7 @@ interface TreeEditorLayoutProps {
   editingNodeId: string | null
   onNodeSelect: (nodeId: string | null) => void
   onSelectAnswerType: (nodeId: string, type: 'decision' | 'action' | 'solution') => void
+  onNodeDelete?: (nodeId: string) => void
   onNodeContextMenu?: (e: React.MouseEvent, nodeId: string) => void
 }
 
@@ -29,6 +30,7 @@ export function TreeEditorLayout({
   editingNodeId,
   onNodeSelect,
   onSelectAnswerType,
+  onNodeDelete,
   onNodeContextMenu,
 }: TreeEditorLayoutProps) {
   const editorMode = useTreeEditorStore(s => s.editorMode)
@@ -72,6 +74,7 @@ export function TreeEditorLayout({
               selectedNodeId={editingNodeId}
               onNodeSelect={onNodeSelect}
               onSelectAnswerType={onSelectAnswerType}
+              onNodeDelete={onNodeDelete}
               onNodeContextMenu={onNodeContextMenu}
             />
           </div>