This adds a temporary GUI/API server during the database migration. It
responds with 200 OK and some log output for every request. This serves
two purposes:
- Primarily, for deployments that use the API as a health check, it
gives them something positive to accept during the migration, reducing
the risk of the migration getting killed halfway through and restarted,
thus never completing.
- Secondarily, it gives humans who happen to try to load the GUI some
sort of indication of what's going on.
Obviously, anything that expects a well-formed API response at this
stage is still going to fail. They were already failing though, as we
didn't even listen at this point before.
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