Hi @mfb,
The App Server doesn’t have built-in support for indices like this, but the description you’ve pasted here gives you a starting point – you can add an index to the underlying table on the expression for that column (in this case app_storage_data.data -> 'kChGi1Zhutg='::text – ideally as a partial index with the constraint app_storage_data.table_id = 5 so you don’t index all the other rows from other tables). This is analagous to what happens on a Dedicated plan when you configure an index.
Of course, at this point you’re messing with your Postgres database by hand, which means that automatic migrations might have trouble (eg if you take a migration that deletes that table, you’ll probably need to delete the index first). I recommend the Postgres docs – there’s a lot, but there’s usually enough information for whatever you’re trying to do!