[19.0][ADD] res_company_admin_sync: keep system administrators assigned to every company

[mikecolangelo]

New module: res_company_admin_sync

In a multi-company database, nothing keeps administrators' company assignments in sync with the companies that exist:

• a company created after the administrator never lands in their company_ids, so it never shows up in their company switcher;
• a user promoted to administrator keeps whatever partial assignment they had.

We audited a real ~270-company production database: every single administrator had accumulated gaps — up to 31 active companies missing each, including the most recently created ones. The administrators technically retained access through base.group_erp_manager exemptions, but the company switcher (which only lists company_ids) made those companies effectively unreachable in the UI.

This module grants automatically instead:

• creating a company adds it to every base.group_system user's companies;
• granting base.group_system adds every existing company to the new administrator.

Deliberately one-way (it never revokes): removing companies on demotion could silently strip access the user still legitimately needs, so trimming is left as an explicit administrative action.

Only depends on base. 4 tests, green on a fresh vanilla Odoo 19 install with only this module.