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/backtracking/0039-combination-sum.org

@@ -1,18 +1,71 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0039. Combination Sum :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0039. Combination Sum][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0039. Combination Sum][0039. Combination Sum]]
 :END:
 
+Given an array of *distinct* integers ~candidates~ and a target integer ~target~, return /a list of all *unique combinations* of /~candidates~/ where the chosen numbers sum to /~target~/./ You may return the combinations in *any order*.
+
+The *same* number may be chosen from ~candidates~ an *unlimited number of times*. Two combinations are unique if the frequency of at least one of the chosen numbers is different.
+
+The test cases are generated such that the number of unique combinations that sum up to ~target~ is less than ~150~ combinations for the given input.
+
+*Example 1:*
+
+
+#+begin_src
+Input: candidates = [2,3,6,7], target = 7
+Output: [[2,2,3],[7]]
+Explanation:
+2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
+7 is a candidate, and 7 = 7.
+These are the only two combinations.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: candidates = [2,3,5], target = 8
+Output: [[2,2,2,2],[2,3,3],[3,5]]
+#+end_src
+
+
+*Example 3:*
+
+
+#+begin_src
+Input: candidates = [2], target = 1
+Output: []
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= candidates.length <= 30~
+
+ - ~2 <= candidates[i] <= 40~
+
+ - All elements of ~candidates~ are *distinct*.
+
+ - ~1 <= target <= 40~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
+        
+    }
+};
 #+end_src