Honor internal: true in customer notification emails

Currently, custom fields marked as internal: true in ticket settings still appear in the system "Your ticket has been submitted" confirmation email sent to customers.

The internal flag controls UI and form visibility correctly, but it does not carry over to the email template renderer. This means sensitive or technical fields like error types, user agents, Sentry events, and auth data end up visible to customers in the submission email even when they are explicitly marked as internal.

Requested behavior: any custom field with internal: true should be excluded from all customer-facing system emails, including the ticket submission confirmation.

Workaround today: teams have to set skipNotifications: true on ticket creation and send their own confirmation email from their backend, which adds unnecessary complexity for a flag that should already signal "don't show this to the customer."