Automated emails: opt-in per booking type instead of opt-out

When creating an automated post-booking email, it should apply to no booking types by default — and I should be able to select directly in the email editor which booking types it applies to.

Currently, automated emails are enabled for all booking types by default. With many booking types, I have to manually disable the email one by one — which is error-prone and easy to forget when adding new booking types later.

Proposed solution:

• Opt-in as default: a new automated email applies to no booking types until actively selected
• Booking type selection directly in the email editor
• Still adjustable per booking type in the Advanced tab (two-way relation)