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/0017-letter-combinations-of-a-phone-number.org

@@ -1,18 +1,52 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0017. Letter Combinations of a Phone Number :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0017. Letter Combinations of a Phone Number][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0017. Letter Combinations of a Phone Number][0017. Letter Combinations of a Phone Number]]
 :END:
 
+Given a string containing digits from ~2-9~ inclusive, return all possible letter combinations that the number could represent. Return the answer in *any order*.
+
+A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+
+*Example 1:*
+
+
+#+begin_src
+Input: digits = "23"
+Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: digits = "2"
+Output: ["a","b","c"]
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= digits.length <= 4~
+
+ - ~digits[i]~ is a digit in the range ~['2', '9']~.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    vector<string> letterCombinations(string digits) {
+        
+    }
+};
 #+end_src