Is your feature request related to a problem? Please describe.
When enabling open registration for public services, there is currently no way to require users to explicitly accept Terms of Service (TOS) before completing registration. This makes it difficult to enforce legal or policy requirements during self-enrollment.

Describe the solution you'd like
Add support for a mandatory "Accept TOS" checkbox in the registration form.

• The checkbox must be checked before a user can successfully register.
• The label text should be configurable by the Rauthy admin.
• Optionally, allow linking the text to an external Terms of Service page (instead of embedding the full text).
  This would allow public-facing services to meet compliance requirements without additional customization.

Describe alternatives you've considered

• Manually modifying the registration page to include a TOS checkbox (not maintainable and would break with updates).
• Relying on external documentation instead of enforcing acceptance (not legally sufficient in many cases).

Additional context
This feature would be especially useful for services that allow open, public registration and need to comply with legal requirements by collecting explicit user consent. It should be optional and configurable so that private/internal deployments are not affected.