Add validation to prevent duplicate domain hosts

[danielnsilva]

What problem will this feature address?

I've run into issues where team members accidentally create duplicate domains, which breaks Traefik routing and causes deployments to fail. Right now there's nothing stopping multiple apps from using the same domain.

Describe the solution you'd like

We should prevent duplicate domains by:

1. Adding a unique constraint at the database level
2. Checking for duplicates before creating/updating domains
3. Showing a error message when someone tries to use an existing domain

Describe alternatives you've considered

For now, manually check existing domains before creating new ones and use naming conventions (e.g., project-alpha-app.domain.com, project-beta-app.domain.com), but this doesn't scale well with multiple team members and is error-prone.

Additional context

No response

Will you send a PR to implement it?

Maybe, need help