bark: Don't store spent VTXOs from the mailbox as spendable

• check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)

Quick recovery fix. An issue users run into after recovery is that bark will replay the mailbox which may contain VTXOs they have previously spent. We can solve this by storing spent VTXOs in the database so we can avoid storing them again.

This shouldn't break cases where the mailbox contains unregistered (but spendable) VTXOs as they will be replayed as normal so bark can later register them before making a payment.