migrate 392 questions to pattern format

backend/data/questions/convert-bst-to-greater-tree.yaml

@@ -6,8 +6,10 @@ leetcode_url: https://leetcode.com/problems/convert-bst-to-greater-tree/
 categories:
   - trees
 patterns:
-  - dfs
-  - tree-traversal
+  - slug: dfs
+    is_optimal: true
+  - slug: tree-traversal
+    is_optimal: false
 
 function_signature: "def convert_bst(root: TreeNode | None) -> TreeNode | None:"