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>

backend/app/core/step_sync.py

@@ -162,7 +162,7 @@ async def sync_steps_from_tree(
                     is_active = true
             """),
             {
-                "title": step_data["title"],
+                "title": step_data["title"][:255],
                 "step_type": step_data["step_type"],
                 "content": json.dumps(step_data["content"]),
                 "created_by": str(resolved_author_id),