Support SMS OTP (One-Time Password) for user registration

Feature request

Which Nextcloud Version are you currently using:
Nextcloud Hub 25 Autumn (32.0.5)

Is your feature request related to a problem? Please describe.
The current registration flow works great with email verification. However, in certain use cases—especially for mobile-centric communities or regions where mobile identity is the primary standard—having more verification options would be very beneficial. Adding SMS support would provide admins with more flexibility to tailor the registration experience to their specific user base.

Describe the solution you'd like
I would like to propose adding SMS One-Time Password (OTP) as an optional verification method.

Key features could include:

1. SMS Verification Option: An administrative toggle to enable SMS-based registration alongside or instead of email.
2. Verification Flow: A simple step where users receive a short code via SMS to verify their phone number during sign-up.
3. Gateway Support: A way to configure SMS providers (like Twilio, Vonage, or a custom API endpoint) within the registration settings.

Describe alternatives you've considered
I have explored using the current email verification, which is excellent for most scenarios. I am simply looking for a way to extend these capabilities to include phone-based verification to support a wider range of identity verification preferences.

Additional context

• This feature would complement Nextcloud's existing focus on security and user choice.
• Implementing rate-limiting for SMS requests would be a good practice to help admins manage costs and prevent misuse.

No files, tests, or entry points are identified. Start by reviewing the existing email-verification registration flow and its settings, then clarify provider configuration, OTP handling, and rate-limiting requirements. Done should mean an agreed SMS registration flow with administrative controls and verification coverage.