Add API to respond with for running backfills

[jrobotham-square]

When an incident occurs it's often really valuable to be able to see what Backfills are currently running in order to:

• Identify ones that may be causing or exacerbating issues.
• Identify ones that could be stopped to reduce the risk.

This PR exposes a new endpoint to surface the currently running backfills.

As part of this change I've also included instructions on starting mysql locally for development and fixed some issues that occur locally when building / running tests on Apple silicon.

Subsequent PR will integrate this endpoint into the UI to create a screen something like: