Subscription onboarding email template

The signup welcome email is now distinct from the subscription onboarding email; new paid customers get a separate message that walks through their plan limits and what to expect on the first invoice.

• Added a subscription onboarding template that ships when an organization moves from trial to a paid plan.
• Implemented delete addon subscription with audit logging so addon removals stay traceable through the same template system.
• Added an update addon subscription status endpoint with the corresponding email notification.
• Improved billing email dispatch with exception handling that no longer drops the message on a malformed contact row.