feat: add simple rate limiter

[satyam-code45]

Description

This PR implements API rate limiting to strengthen the system’s security and prevent abuse.
It introduces a middleware that restricts the number of requests a user can make within a defined time window.
Stricter limits are applied to sensitive authentication endpoints to mitigate brute-force attacks.

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Semver Changes

• Minor (new features, no breaking changes)
• Patch (bug fix, no new features)
• Major (breaking changes)

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Issues

Closes #18 — [Feature]: Add API Rate Limiting

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Checklist

• I have read the Contributing Guidelines.

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🧩 Summary

• Added rate-limiting middleware for all API routes.
• Implemented stricter limits for /api/auth/login and /api/auth/register.
• Returns 429 Too Many Requests when the limit is exceeded.
• Enhances protection against brute-force and spam attacks.

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🧪 How to Test

1. Start the server locally.
2. Make repeated requests to /api/auth/login and /api/auth/register.
3. Observe that after exceeding the allowed number of requests, the API responds with 429 Too Many Requests.
4. Verify that normal routes still function within general rate limits.

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Labels: security · enhancement · middleware
Related PR: #25