Booking page example: reliable confirmations for signed-in users

The built-in booking page example incorrectly reported "Couldn't submit booking" when a signed-in user booked, because the confirmation email helper only accepts anonymous rows. Submitter confirmations and reminders are now gated to anonymous visitors, and the core booking save is separated from best-effort side-effects so a follow-up failure never marks a saved booking as failed. Since the AI uses this example as a template, the pattern propagates to new apps.