migrate 392 questions to pattern format

backend/data/questions/accounts-merge.yaml

@@ -8,8 +8,10 @@ categories:
   - hash-tables
   - strings
 patterns:
-  - union-find
-  - dfs
+  - slug: union-find
+    is_optimal: true
+  - slug: dfs
+    is_optimal: false
 
 function_signature: "def accounts_merge(accounts: list[list[str]]) -> list[list[str]]:"