Expert 401 responses cause platform logout

### Current Behavior

Recently, the auth method for FF Expert backend was changed. As a result, calls to the Expert BE that are not permitted now receive a 401.

Due to the common interceptor in `frontend/src/api/client.js` this causes `logout` to be called.

### Expected Behavior

A 401 from an external service should not logout the user

### Steps To Reproduce

Operate the expert on a staging env (since this is currently in limbo, you will see that)

### 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