Error when regenerating device credentials

### Current Behavior

Capturing this error Steve hit once when attempting to regenerate device credentials. Tried a second time and it worked. Looks like a timing issue in the clear-up of the old BrokerClient object for the device before generating the new one
```
Error
at async BrokerClient.create (/usr/src/forge/app/node_modules/sequelize/lib/model.js:1362:12)
at async Object.createClientForDevice (/usr/src/forge/app/node_modules/@flowfuse/flowfuse/forge/db/controllers/BrokerClient.js:100:13)
at async model.refreshAuthTokens (/usr/src/forge/app/node_modules/@flowfuse/flowfuse/forge/db/models/Device.js:181:36)
at async Object. (/usr/src/forge/app/node_modules/@flowfuse/flowfuse/forge/routes/api/device.js:645:29)
```

```
"ValidationErrorItem","message":"username must be unique","stack":"","path":"username","value":"device:[ABC]:[XYZ]"
```

### Expected Behavior

_No response_

### Steps To Reproduce

_No response_

### Environment

- FlowFuse version:
- Node.js version:
- npm version:
- Platform/OS:
- Browser:

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate