Add user_enable_two_factor email notifications and audit events

What does this MR do and why?

Adds email notifications and audit events when a user registers a OTP authenticator or WebAuthn device.

• Creates the user_enable_two_factor audit event type

References

• Add email notifications & audit events when a u... (#549619 - closed)

Screenshots or screen recordings

OTP	WebAuthn

Audit event entry:

How to set up and validate locally

1. Enable two-factor authentication for an account
2. Review sent email in /rails/letter_opener/
3. View Audit events for entry

For Enterprise Users:

1. Simulate a SaaS instance
2. Create a top-level group and assign it a Premium subscription
3. Verify a domain
   • Add group domains

   domain = PagesDomain.last
   domain = update!(
     verified_at: Time.current,
     enabled_until: 1.year.from_now
   )
   ::Groups::EnterpriseUsers::BulkAssociateByDomainWorker.perform_async(domain.id)

4. Re-do the above instructions

MR acceptance checklist

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