fix: add rate limiting to auth routes

[anmolsah]

Description

Added rate limiting protection to authentication routes to prevent brute-force attacks and signup spam. Implemented express-rate-limit middleware on /api/auth/login and /api/auth/signup endpoints.

🔗 Related Issue

Closes #33

Changes Made

• Installed and configured express-rate-limit
• Added rate limiter middleware for login and signup routes
• Configured limit to 10 requests per 15 minutes per IP
• Added proper HTTP 429 response with clear error message
• Kept normal authentication flow unaffected for valid users

Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue