migrate 392 questions to pattern format

backend/data/questions/allocate-mailboxes.yaml

@@ -9,7 +9,8 @@ categories:
   - sorting
   - math
 patterns:
-  - dynamic-programming
+  - slug: dynamic-programming
+    is_optimal: true
 
 function_signature: "def min_distance(houses: list[int], k: int) -> int:"