Global session object is bad practice

[JBorrow]

Right now we are using a global SQLAlchemy Session object for all transactions:

SessionMaker = sessionmaker(bind=engine, autocommit=False, autoflush=False)
session = SessionMaker()

This should be changed so we use SesssionMaker() within each request using Depends in FastAPI: https://fastapi.tiangolo.com/tutorial/sql-databases/.

It may also be worth migrating to SQLModel (which uses pydantic & SQLAlchemy).

[JBorrow]

@plaplant Let me know your thoughts on this if any, otherwise I will just go ahead and merge