feat: populate note files with problem descriptions and code stubs

Add populate-notes.mjs that fetches problem descriptions and
Python/C++ code stubs from LeetCode's GraphQL API. Populated
all 197 NeetCode 150 note files with:
- Problem description (examples, constraints)
- Python code stub (function signature)
- C++ code stub (function signature + includes)

API responses cached in leetcode/.cache/leetcode/ for instant re-runs.

org/study_deck_02/dsa/1-d-dynamic-programming/0647-palindromic-substrings.org

@@ -1,18 +1,56 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0647. Palindromic Substrings :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0647. Palindromic Substrings][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0647. Palindromic Substrings][0647. Palindromic Substrings]]
 :END:
 
+Given a string ~s~, return /the number of *palindromic substrings* in it/.
+
+A string is a *palindrome* when it reads the same backward as forward.
+
+A *substring* is a contiguous sequence of characters within the string.
+
+*Example 1:*
+
+
+#+begin_src
+Input: s = "abc"
+Output: 3
+Explanation: Three palindromic strings: "a", "b", "c".
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: s = "aaa"
+Output: 6
+Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= s.length <= 1000~
+
+ - ~s~ consists of lowercase English letters.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def countSubstrings(self, s: str) -> int:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    int countSubstrings(string s) {
+        
+    }
+};
 #+end_src