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/FlowCanvas.tsx

@@ -35,10 +35,11 @@ interface FlowCanvasProps {
   selectedNodeId: 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
 }
 
-function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onNodeContextMenu }: FlowCanvasProps) {
+function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onNodeDelete, onNodeContextMenu }: FlowCanvasProps) {
   const { fitView, setCenter } = useReactFlow()
   const { nodes: layoutNodes, edges: layoutEdges, collapsedNodeIds, toggleCollapse, onNodesMeasured } = useTreeLayout(selectedNodeId)
   const [minimapVisible, setMinimapVisible] = useState(true)
@@ -51,7 +52,7 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onN
         return {
           ...n,
           selected: n.id === selectedNodeId,
-          data: { ...data, onToggleCollapse: toggleCollapse, onContextMenu: onNodeContextMenu },
+          data: { ...data, onToggleCollapse: toggleCollapse, onDelete: onNodeDelete, onContextMenu: onNodeContextMenu },
         }
       }
       if (n.type === 'answerStub') {
@@ -64,7 +65,7 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onN
       }
       return n
     })
-  }, [layoutNodes, selectedNodeId, toggleCollapse, onSelectAnswerType, onNodeContextMenu])
+  }, [layoutNodes, selectedNodeId, toggleCollapse, onSelectAnswerType, onNodeDelete, onNodeContextMenu])
 
   const [nodes, setNodes, onNodesChange] = useNodesState(nodesWithCallbacks)
   const [edges, setEdges, onEdgesChange] = useEdgesState(layoutEdges)