Include `username` in API logs

In the case of authenticated queries from API, include the username in the API logs for debugging purposes.

For example, API log when a user subscribes to a channel looks like this:
```
kernelci-api | INFO: 172.20.0.1:42984 - "POST /latest/subscribe/node HTTP/1.1" 200 OK
```

That should contain `username` of requesting user:
```
kernelci-api | INFO: 172.20.0.1:42984 - staging.kernelci.org "POST /latest/subscribe/node HTTP/1.1" 200 OK
```

Start at the API request logging path for authenticated queries, using POST /latest/subscribe/node as the concrete entry point. Verify that the completed log includes the requesting username while preserving the existing request details, and exercise that subscription request to confirm the output.

Written by the indexing model from the issue text.