Expose OTP cooldown retry time in send-otp response

[subhash-22-codes]

Endpoint:
POST /send-otp

Problem:
The current cooldown response returns a static message and does not expose remaining retry duration.

Because of this, frontend cannot show an accurate resend countdown timer.

Expected:

• calculate remaining cooldown duration
• expose retry metadata in response
• frontend should be able to consume this value for resend UX

Possible files:

• backend/app/api/auth.py
• frontend/src/pages/Register.tsx