migrate 392 questions to pattern format

backend/data/questions/subsets-ii.yaml

@@ -8,7 +8,8 @@ categories:
   - sorting
   - recursion
 patterns:
-  - backtracking
+  - slug: backtracking
+    is_optimal: true
 
 function_signature: "def subsets_with_dup(nums: list[int]) -> list[list[int]]:"