Use SQLModel Session for better typehinting

[piercefreeman]

Mountaineer's database support has focused on using a SQLAlchemy AsyncSession for database connections alongside SQLModel for query construction. This was due to some initial limitations in the async session provided by SQLModel and SQLAlchemy2.

Mixing the sessions works but can result in some clunky code. Each database request has to explicitly get the scalars for typehinting to resolve in mypy:

query_result = await db_session.execute(query)
result = query_result.scalars().all()

Additionally, there are some cases where this approach results in the wrong typehints - like when we're trying to fetch a tuple of field:

query = select(models.ModelA, models.ModelB)

We expect this would result in a tuple of (model_a, model_b) as the typehint suggests, but it only gathers the first object since scalars() just arbitrarily takes the first value.

To address these shortcomings, we switch to using the typehints provided by SQLModel. Technically we use a thin wrapper on top of AsyncSession in order to support text queries like discussed here:
fastapi/sqlmodel#909

Old .execute requests are still supported, but clients are advised to switch to .exec to get the proper typehints and to avoid the deprecation warning.