Improved onboarding for OnCall users

We’ve streamlined the onboarding process for OnCall, making it quicker and easier for new users to get started.

Now, new users can access OnCall immediately without waiting for an admin to log in. OnCall initialization has been moved to the backend, reducing setup time and minimizing potential errors. Additionally, user synchronization between Grafana and OnCall is significantly faster, so newly created users can start using OnCall within minutes—without any delays or extra steps.