Update to Pydantic 2

[scy]

Pydantic 1 is still supported with current versions of FastAPI and SQLModel, but starts to throw warnings in Python 3.13, cf. 73174ad.

However, Pydantic 2 also brings a couple of changes that might need addressing with some of our models. Also, we're currently limited to SQLModel < 0.0.11 (see 706462b), and SQLModel doesn't support Pydantic 2 before 0.0.12. In other words, we'll need to update SQLModel (and our models) first, which will need some SQLAlchemy knowledge.