Add API Endpoint for Last Updated Timestamp

[jagdish-15]

Currently, there is no way to determine when the leaderboard data was last updated. Since all leaderboard JSON files (overall, monthly, weekly, daily) are updated together via workflow, we can expose the last modified timestamp of one of these files through a new API endpoint.

Expected Behavior

• Create a new API endpoint: /api/last-updated
• The endpoint should return the last modified time of one of the data files (e.g., data/overall.json)
• Response should be in JSON format

Example response:

{
  "lastUpdated": "2026-04-07T10:30:00.000Z"
}