Use email otp as verification fallback for 2fa

What does this MR do and why?

Allow user to use email OTP as fallback verification for 2fa.

Part 2 for https://gitlab.com/gitlab-org/gitlab/-/issues/570174.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/570174+s

Screenshots or screen recordings

Scenario	Before	After	Note
User only has 2FA enabled, but has email_otp_required_after set to nil	unchanged	Screen_Recording_2025-11-03_at_3.11.50_PM	the send email OTP link in the footer is not available because email_otp_required_after value does not satsify permission ❌
User only has 2FA enabled, but has email_otp_required_after set to the future	unchanged	Screen_Recording_2025-11-03_at_3.13.40_PM	the send email OTP link in the footer is not available because email_otp_required_after value does not satisfy permission ❌
User only has 2FA enabled, but has email_otp_required_after set to the past	email otp fallback is not available	Screen_Recording_2025-11-03_at_3.05.26_PM	the send email OTP link in the footer is available because email_otp_required_after value satisfies permission ✅
User has webAuthn enabled, but chooses to verify with 2FA while having email_otp_required_after set to nil	unchanged	Screen_Recording_2025-11-03_at_3.16.36_PM	send email OTP link in the footer is not available because email_otp_required_after value does not satisfy permission ❌
User has webAuthn enabled, but chooses to verify with 2FA while having email_otp_required_after set to the past	email otp fallback is not available	Screen_Recording_2025-11-03_at_12.55.15_PM	the send email OTP link in the footer is available because email_otp_required_after value satisfies permission ✅

How to set up and validate locally

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