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/pages/TreeEditorPage.tsx

@@ -815,6 +815,7 @@ export function TreeEditorPage() {
           editingNodeId={editorAI.isOpen ? null : editingNodeId}
           onNodeSelect={handleNodeSelect}
           onSelectAnswerType={handleSelectAnswerType}
+          onNodeDelete={deleteNode}
           onNodeContextMenu={editorAI.openContextMenu}
         />
       </div>