feat(auth): migrate OTP storage from MongoDB to Redis TTL

[amandotyadav]

Summary

This PR migrates OTP storage from MongoDB user documents to Redis using native TTL support.

closes #131

Changes Made

• Added Redis-based OTP storage
• Implemented OTP expiration using Redis TTL
• Removed manual OTP timestamp validation logic
• Deleted OTPs from Redis after successful verification
• Refactored OTP handling into a dedicated service layer
• Preserved existing authentication API behavior and response formats

Benefits

• Reduces unnecessary MongoDB writes
• Separates temporary OTP state from persistent user data
• Simplifies OTP expiration handling
• Improves scalability and performance
• Provides a cleaner production-grade authentication architecture

Backward Compatibility

• Existing signup flow remains unchanged
• Existing verify OTP flow remains unchanged
• Existing resend OTP flow remains unchanged
• No frontend changes required

Testing

• Verified OTP generation and storage in Redis
• Verified OTP expiration through Redis TTL
• Verified OTP verification flow
• Verified OTP deletion after successful verification
• Verified resend OTP flow
• Confirmed existing authentication behavior remains intact

[vercel]

@amandotyadav is attempting to deploy a commit to the Sunil Kumar's projects Team on Vercel.

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[chthonn]

LGTM