Add audit events for Pages Domains lifecycle

What does this MR do and why?

Add audit events for Pages Domains lifecycle to allow group, project, or instance administrators to have an audit trail for domain management. This is especially important for .com as verified domains are used for Enterprise users.

References

• #545633

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Simulate SaaS GITLAB_SIMULATE_SAAS=1
2. Enable pages with custom domains
3. Enable the Let’s Encrypt integration
4. Ensure the group has a subscription
5. Run the following snippet to simulate all events and emit audit events in a project
   • Click to expand

       user = User.first
       group = Group.first
       
       # Create a project
       attrs = { path: 'domain-verification', namespace_id: group.id }
       project = ::Projects::CreateService.new(user, attrs).execute
       
       # Stub the TXT lookup instead of hitting real DNS
       def verify(domain, user, dns:)
         svc = VerifyPagesDomainService.new(domain, user)
         svc.define_singleton_method(:dns_record_present?) { dns }
         svc.execute
       end
       
       def create(project, user, host)
         Pages::Domains::CreateService.new(project, user, domain: host).execute
       end
       
       # pages_domain_created
       domain = create(project, user, 'example.com')
       
       # pages_domain_updated  (empty params is a valid no-op update that still persists)
       Pages::Domains::UpdateService.new(project, user, {}).execute(domain)
       
       # pages_domain_deleted
       Pages::Domains::DeleteService.new(project, user, nil).execute(domain)
       
       # pages_domain_enabled
       to_enable = create(project, user, 'enabled.example.com')
       verify(to_enable, user, dns: true)
       
       # pages_domain_verification_succeeded: already enabled, but unverified
       to_reverify = create(project, user, 'succeeded.example.com')
       to_reverify.update_columns(verified_at: nil, enabled_until: 1.week.from_now)
       verify(to_reverify, user, dns: true)
       
       # pages_domain_verification_failed: verified, still in grace period, stops resolving
       verified = create(project, user, 'verified.example.com')
       verified.update_columns(verified_at: Time.current, enabled_until: 1.week.from_now)
       verify(verified, user, dns: false)
       
       # pages_domain_disabled: expired domain that no longer resolves
       expired = create(project, user, 'expired.example.com')
       expired.update_columns(verified_at: 1.week.ago, enabled_until: 1.day.ago)
       verify(expired, user, dns: false)

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