Audit event for model creation

What does this MR do and why?

Logs self-hosted model creation as audit events. Refactoring creation to a service helps centralising the logic.

How to set up and validate locally

1. on gdk rails c, execute the service:

   Ai::SelfHostedModels::CreateService.new({name: "my model 3", model: "codestral", endpoint: "http://localhost.com", user: User.find_by_id(1)}).execute

2. On gdk psql, verify that an event was created

   select * from audit_events order by created_at desc;

Related to #477999 (closed)