Element-R: Unable to verify a device which has outstanding outgoing secret requests

STR:
* Sign in on a new device
* Verify against a device which, for some reason, does not have a copy of the private cross-signing keys. (It's not entirely clear how to get into this situation; one way to do it might be to verify against another device which doesn't correctly implement secret sharing, or one which falls off the internet just after verifying. Alternatively, https://github.com/element-hq/element-web/issues/27806).
* The new device is not verified, and nor does it receive the cross-signing keys. So far, this is https://github.com/element-hq/element-web/issues/27655.
* Verify again, against a properly-verified device.

It still doesn't work. In other words, the new device was permanently broken by the attempt to verify against the broken device. This is a *very* poor experience.

In the console, there is this error:

```
17:06:15.181 ERROR matrix_sdk_crypto::machine: Error handling a verification event: Backend(DomException { code: 0, name: "ConstraintError", message: "A mutation operation in the transaction failed because a constraint was not satisfied." })
at /home/rav/work/matrix-rust-sdk/crates/matrix-sdk-crypto/src/machine.rs:1095
in matrix_sdk_crypto::machine::receive_to_device_event with sender="@a:xps9320.sw1v.org" event_type="m.key.verification.done"
in matrix_sdk_crypto::machine::receive_sync_changes
```

I think what's happening is that the database already contains an outgoing secret request; then when the new verification succeeds, we try to create a new secret request. This causes the database consistency error, and, since we're trying to persist the verification state in the same transaction, that also fails.