migrate 392 questions to pattern format

backend/data/questions/trapping-rain-water.yaml

@@ -8,8 +8,10 @@ categories:
   - two-pointers
   - stack
 patterns:
-  - two-pointers
-  - monotonic-stack
+  - slug: two-pointers
+    is_optimal: false
+  - slug: monotonic-stack
+    is_optimal: true
 
 function_signature: "def trap(height: list[int]) -> int:"