migrate 392 questions to pattern format

backend/data/questions/3sum-with-multiplicity.yaml

@@ -8,7 +8,8 @@ categories:
   - hash-tables
   - two-pointers
 patterns:
-  - two-pointers
+  - slug: counting-sort
+    is_optimal: true
 
 function_signature: "def three_sum_multi(arr: list[int], target: int) -> int:"