fix: prevent circular parent_node_id in KB troubleshooting import

AI-generated trees can have circular next_node_id references (e.g.,
node A → B → A). The parent mapping now checks for cycles before
assigning parent_node_id, preventing FK deadlocks during insert.

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

backend/app/core/kb_conversion_service.py

@@ -284,17 +284,30 @@ def _parse_troubleshooting_response(
         if nid not in node_id_to_parent:
             node_id_to_parent[nid] = None  # default: no parent
 
-    # Trace parent relationships
+    # Trace parent relationships (only set if it won't create a cycle)
+    def _would_cycle(child: str, parent: str) -> bool:
+        """Check if setting child's parent to parent creates a cycle."""
+        visited: set[str] = set()
+        cur: str | None = parent
+        while cur:
+            if cur == child:
+                return True
+            if cur in visited:
+                break
+            visited.add(cur)
+            cur = node_id_to_parent.get(cur)
+        return False
+
     for node in raw_nodes:
         nid = node.get("id", "")
         # Options point to children
         for opt in node.get("options", []):
             child_id = opt.get("next_node_id")
-            if child_id and child_id in node_id_to_data:
+            if child_id and child_id in node_id_to_data and not _would_cycle(nid, child_id):
                 node_id_to_parent[child_id] = nid
         # next_node_id points to child
         next_id = node.get("next_node_id")
-        if next_id and next_id in node_id_to_data:
+        if next_id and next_id in node_id_to_data and not _would_cycle(nid, next_id):
             node_id_to_parent[next_id] = nid
 
     # Create import node records preserving order