trust: add ExternalHeaderTearDown

### Confirmation

- [x] I have checked for similar issues.

### Problem

As per the [go-version of TBC](https://github.com/hemilabs/heminetwork/blob/40da4645e99ca23b3e8e63f0463e8ed71d52bdfa/service/tbc/tbc.go#L3344), add a tear down step to `trust`, that closes the database.

### Suggested solution

Follow the go implementation.

### Additional Information

_No response_