migrate 392 questions to pattern format

backend/data/questions/palindromic-substrings.yaml

@@ -7,8 +7,10 @@ categories:
   - strings
   - dynamic-programming
 patterns:
-  - two-pointers
-  - dynamic-programming
+  - slug: two-pointers
+    is_optimal: true
+  - slug: dynamic-programming
+    is_optimal: false
 
 function_signature: "def count_substrings(s: str) -> int:"