Implement Email OTP Verification

[Hamzaa6296]

Description:

What needs to be done

• Setup Nodemailer for sending emails
• Generate 6-digit OTP on signup
• Store OTP with expiry time in database
• Create POST /auth/verify-otp endpoint
• Send verification email on signup
• Block unverified users from login

Acceptance Criteria

• OTP email sent on signup
• OTP expires after 10 minutes
• Verified flag set to true after correct OTP
• Unverified users cannot login