Add school email domain API

[PetarSimonovic]

• Expose endpoints for school owners to list and add email domains for their school
• Validate domains before saving (format, uniqueness per school)
• Restrict access so only authorised users can manage domains for their own school
• After a domain is added, sync the full domain list to Profile
• Return a simple list of domain strings for the UI to display
• Cover happy path and failure cases with request specs (invalid domain, duplicate, unauthorised user)