Implement rate limiting to prevent abuse and ensure service stability

Fixes #40

Implement rate limiting to prevent excessive requests and ensure service stability.

* Add `express-rate-limit` middleware to `pages/api/exam-result.js` and `pages/api/scholarship-data.js` to limit requests to 100 per 15 minutes per IP address.
* Handle blocked requests gracefully by returning a 429 status code with a custom message in both API files.
* Update `package.json` to include `express-rate-limit` dependency.
* Add logic to `pages/college_predictor.js` and `pages/scholarships_result.js` to handle 429 status code and display a "Rate limit exceeded" message to the user.

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