migrate 392 questions to pattern format

backend/data/questions/array-nesting.yaml

@@ -6,7 +6,8 @@ leetcode_url: https://leetcode.com/problems/array-nesting/
 categories:
   - arrays
 patterns:
-  - dfs
+  - slug: dfs
+    is_optimal: true
 
 function_signature: "def array_nesting(nums: list[int]) -> int:"