Better handling for undeliverable subscribers

We've improved how Buttondown handles undeliverable subscribers:

• Added detailed undeliverability reasons to help you understand why specific subscribers can't receive emails
• New filtering options in the subscribers list to find subscribers by undeliverability status and reason
• API support for retrieving and filtering by undeliverability data
• Improved internal tracking to better identify potentially problematic email addresses

This helps you maintain a cleaner list and provides more insight into delivery problems before they affect your sending reputation.