This update makes a few internal improvements to how VoIP registration settings are initialized and maintained. It helps keep the calling registration process clearer and more consistent, with minimal direct impact for users.

In this commit:
+ Make the expiration interval a field and document it
+ Set the initial value of `state` in `_created` hook instead of via a compute
+ Fix single quotes

This update simplifies how several Odoo Enterprise apps handle internal data replacement behavior. It should make the underlying code easier to maintain without changing day-to-day user workflows.

* = approvals, documents, sign, voip

Distinction between "replace" and "insert-and-replace" can be guessed based on
the type of the provided data.

task-2957295

https://github.com/odoo/odoo/pull/98404