Draft: Use the proper worker for background migrations

What does this MR do and why?

With the switch to multiple databases, background migrations are intended to be scheduled against main or ci, which indicates where they tracking data will live, and which worker will process them. Up until now, we've continued to schedule against main only, since we still have only a single database.

Now that the tooling is merged to control how migrations run with multiple databases, we can add the final wiring which will select the proper worker/tracking database for the background migration.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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