Potential fix for code scanning alert no. 1: Missing rate limiting

[mochiron-desu]

Potential fix for https://github.com/EpicBotCoders/leetDiscord/security/code-scanning/1

To fix the problem, the /api/stats route should be protected by an express-rate-limit middleware, just like the existing frontendLimiter. This constrains how frequently a single client can trigger the database queries, reducing the risk of denial‑of‑service via repeated calls.

Concretely, in index.js we should:

1. Define a new rate limiter instance specifically for the /api/stats route (e.g., statsLimiter), using RateLimit that is already required at line 104.
2. Apply this limiter to the route by adding it as middleware in the app.get('/api/stats', ...) definition: app.get('/api/stats', statsLimiter, async (req, res) => { ... });.
3. Place the new limiter definition near the existing frontendLimiter so configuration is easy to find and consistent.

No new imports or external helpers are required; we reuse the already imported express-rate-limit. Functionality of the handler itself is unchanged; only middleware is added to control access frequency.

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