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/0078-subsets.org

@@ -1,18 +1,54 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0078. Subsets :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0078. Subsets][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0078. Subsets][0078. Subsets]]
 :END:
 
+Given an integer array ~nums~ of *unique* elements, return /all possible/ /subsets/ /(the power set)/.
+
+The solution set *must not* contain duplicate subsets. Return the solution in *any order*.
+
+*Example 1:*
+
+
+#+begin_src
+Input: nums = [1,2,3]
+Output: [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: nums = [0]
+Output: [[],[0]]
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= nums.length <= 10~
+
+ - ~-10 <= nums[i] <= 10~
+
+ - All the numbers of ~nums~ are *unique*.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def subsets(self, nums: List[int]) -> List[List[int]]:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    vector<vector<int>> subsets(vector<int>& nums) {
+        
+    }
+};
 #+end_src