[FastAPI] SQLite performance needs fixing

With the addition of a5943bf2, SQLite's performance has been crushed to smithereens.

There's something about the transaction context taking a lot longer when SQLite is used. We really need to figure this out and address this, as we want all of our supported DB backends to work at expected speeds.

NOTE: This has been temporarily addressed, however, we need to move over to using sqlite:///:memory: as @jelle has recommended; we agree!

Tasks:

• "Fix" sqlite performance issues - 7bfc2bf9
  • This is not a good fix. We are no-oping fsync; which is fine for us currently because nothing externally modifies the database during tests, but we should not assume this will always be the case.
• Migrate to using sqlite:///:memory: for testing