Set up initial backend project skeleton

[digitaldogsbody]

Using Poetry as a dependency manager, create a new project with Flask, FastAPI, SQLAlchemy, Alembic, Pydantic, etc and set up test framework

[digitaldogsbody]

Done in https://github.com/datacite/saluki/commit/152f896ff389cac5e0ec33d230f3319ccfba2432