Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -458,6 +458,7 @@ _**Note:** Our group is currently taking a break from grinding leetcode. The tab
 | September 18, 2025 | [3408](https://leetcode.com/problems/design-task-manager/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3408) | 🟡 Medium |
 | September 19, 2025 | [3484](https://leetcode.com/problems/design-spreadsheet/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3484) | 🟡 Medium |
 | September 20, 2025 | [3508](https://leetcode.com/problems/implement-router/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3508) | 🟡 Medium |
+| September 21, 2025 | [1912](https://leetcode.com/problems/design-movie-rental-system/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1912) | 🔴 Hard |
 
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