Refactors the entire backend to rely on an adapter class

[wpfl-dbt]

We're going to want to play with different backends. We therefore want to collect existing functionality into a single backend adapter that's used throughout the frontend via an injected client.

Warning

This PR has become unbelievably large. In this refactor I'm moving fast and breaking things (agreed by Leo) because I'm working alone, and because this is in service of making something that's easier to test and work with going forward. Much of what's shifting about here is going to get ripped out in the next PR.

Normally I'd turn this into a bunch of smaller PRs, I swear.

Changes proposed in this pull request

• Created new backend abstract class
• Created new PostgreSQL backend child of that class
• Moved all existing logic into this new PostgreSQL class
• Replaced all client-side functions with calls to this class

Guidance to review

• Review the overall architectural arrangement of the new classes and confirm the API feels intuitive enough to extend
• Confirm the unit tests pass. If the tests pass, the code is solid

Relevant links

• Backend API documentation/ideation

Checklist:

• My code follows the style guidelines of this project
• New and existing unit tests pass locally with my changes