fix(#51): fixed type hints of models

[OliveraReeves]

No description provided.

[George-D-S]

Sorry, I think I've lost track of which issue these changes are trying to address.

I did wonder if it was neccesary to have aliases for each case, as unlike how it used to be with Oracle, SQL Server is configured to use case insensitive object names.

[OliveraReeves]

This came up in updating nhsbt_import, where it was missing required fields, but when checking the db some fields could be nullable, so this was just to align the models with what is in the database

[George-D-S]

Ahh right - I see now what's actually changed. That seems fine.

It was having...

    SURNAME: Mapped[Optional[str]] = mapped_column(String(50))
    surname: Mapped[Optional[str]] = synonym("SURNAME")

That puzzled me, but as it's already like that I'll approve the PR.