Add an environment variable to disable the book caching that was added in T265660.
This will mean we can tag a new release and push to production with no caching enabled, so we can collect generation duration statistics for a while before we enable caching.
I have checked that setting APP_ENABLE_CACHE=false will disable caching.
I used this to turn the cache off and on again, to simulate what we might need to do in production when we enable caching, but nothing bad appeared to happen.
This is only something the developers can do, not something a user has control over.
We've enabled caching in production. This saw a bit of an increase in the disk usage (approx. 5GB) which put us at about 90% full. To prevent this, we've added a block storage volume and moved the cache files to it. It's currently 10GB, but we can increase it as required. I'll keep an eye on how much is used (we get notified in the daily log-dump emails).
We are now able to disable & enable caching on production. I'm marking this work as Done.