Move from call_batch_update.py to cron.ts

[foolo]

There is a script src/util/call_batch_update.py which is currently run with a cronjob on our external google cloud VM instance.
The script in turn makes a request to the /api/batch_update API endpoint, which then does the batch update.
We could simplify all this to use src/util/cron.ts instead

Rough idea:

• extract the code in src/app/api/batch_update/route.ts to a util function
• set up a cronjob in src/app/api/batch_update/route.ts that executes that function
• remove the src/app/api/batch_update/route.ts, endpoint. We no longer need this in the API
• remove src/util/call_batch_update.py
• we may now be able to shut down the cloud VM instance, if it's not used for anything else