Adds Ecuador-specific support for the ATS (Anexo Transaccional Simplificado) tax reporting process required by the SRI. This helps Ecuadorian businesses capture retention invoices, tax codes, identification types, and related reference data needed for local compliance reporting.

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:
## 

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Website editors can now identify and search for pages that are not shown in navigation menus. This helps teams manage hidden or shared website pages more reliably, including across multi-website setups.

When a page is not in the menu, it's sometimes difficult to remember its name, this is why we add a boolean field in_menu. This feature takes into account the shared pages in multi websites.

When comparing website pages with the master version, links now point to the matching page in the current version. This makes version comparisons easier to navigate and reduces confusion for users reviewing website changes.

To have the link to the pages of the current verison when you clicked on Diff with Master(Technical Feature).

Quotation emails are now sent to the customer's primary contact instead of the invoice address. This helps ensure sales documents reach the right person, reducing missed communications and follow-up delays.

Task: https://www.odoo.com/web#id=28048&view_type=form&model=project.task&action=333&active_id=248&menu_id=4720

This update removes legacy API support and completes the move to Odoo's newer internal framework. It affects many core and add-on areas, improving maintainability and consistency but requiring older custom integrations or modules to be updated.

The purpose of this branch is to drop the backward compatibility with the old API:
- remove method wrappers;
- convert all the remaining old-API calls to the new API;
- remove the processing of `_columns`, `_defaults`, `_inherit_fields`, `_all_columns`;
- remove all old-API fields;
- implement sparse fields:

``` python
stuff = fields.Serialized()
value = fields.Float(sparse='stuff')
```
- modify the registry to become a mapping `model_name -> model_class`;
- adapt XML-RPC and JSON-RPC gateways.

Subscriptions and subscription templates are now handled as separate business records, reducing confusion around company, pricing, status, and renewal settings. This also improves subscription dashboards, website views, demo data, and automated end-date handling so teams can manage contracts more reliably.

Task: https://www.odoo.com/web#id=24244&view_type=form&model=project.task&action=327&menu_id=4720
Pad: https://pad.odoo.com/p/r.bdc128dd769036b9230ed8d8015c8d6e