Test asynchronous verification with GCP downscaling

[manuelwedler]

With the new server architecture in API v2, we will use multithreading for background verification. We found that potentially GCP Cloud Run could kill a server instance that has no open requests but runs a verification in the background (#1830 (comment)).

To make sure API v2 works as expected, we should trigger the downscaling on staging GCP deployment and see if verification still succeeds when instances are killed, or if jobs are kept in pending state in the db and we cannot recover.

Since we need to deploy API v2 to GCP for testing this, this issue is a last step of the API v2 milestone before we can publish to master.