API: Change OTP code from alphanumeric to numeric

[rajdip-b]

Description

Currently, the API sends an OTP while signing in or signing up with an email address. The problem isl, as per the standards, we should be sending a numeric OTP. But currently, it is alphanumeric. We would like to make this change.

Solution

• Change the OTP generation method to generate a 6-digit numeric code.

Additional context

Make sure the tests pass. You won't need to add any other tests for this issue.