feat: implement MongoDB integration to user login and register.

[Shreyanshi210205]

Description

This update integrates MongoDB into the project using Mongoose. A database connection has been established to replace the previous in-memory user storage. The change enables persistent user data storage for registration, login, and authentication features.
User schema is created in models/User.js with unique email and password .

Semver Changes

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

Issues

#33

Checklist

• I have read the Contributing Guidelines.