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/math-geometry/0840-magic-squares-in-grid.org

@@ -1,18 +1,64 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0840. Magic Squares In Grid :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0840. Magic Squares In Grid][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0840. Magic Squares In Grid][0840. Magic Squares In Grid]]
 :END:
 
+A ~3 x 3~ *magic square* is a ~3 x 3~ grid filled with distinct numbers *from *1* to *9 such that each row, column, and both diagonals all have the same sum.
+
+Given a ~row x col~ ~grid~ of integers, how many ~3 x 3~ magic square subgrids are there?
+
+Note: while a magic square can only contain numbers from 1 to 9, ~grid~ may contain numbers up to 15.
+
+*Example 1:*
+
+
+#+begin_src
+Input: grid = [[4,3,8,4],[9,5,1,9],[2,7,6,2]]
+Output: 1
+Explanation: 
+The following subgrid is a 3 x 3 magic square:
+
+while this one is not:
+
+In total, there is only one magic square inside the given grid.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: grid = [[8]]
+Output: 0
+#+end_src
+
+
+*Constraints:*
+
+ - ~row == grid.length~
+
+ - ~col == grid[i].length~
+
+ - ~1 <= row, col <= 10~
+
+ - ~0 <= grid[i][j] <= 15~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def numMagicSquaresInside(self, grid: List[List[int]]) -> int:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    int numMagicSquaresInside(vector<vector<int>>& grid) {
+        
+    }
+};
 #+end_src