Avoid sa_column where possible

[jpmckinney]

It can cause programmer errors like in #235.

• [x] DECIMAL: https://sqlmodel.tiangolo.com/advanced/decimal/#decimals-in-sqlmodel
• needed for DateTime(timezone=True): https://github.com/tiangolo/sqlmodel/issues/539
• needed for JSON https://github.com/tiangolo/sqlmodel/issues/42 (<class 'dict'> has no matching SQLAlchemy type if sa_type=JSON is omitted)
• [x] DateTime(timezone=False) (maybe better to stay consistent with DateTime(timezone=True))

[jpmckinney]

When making changes to Decimal in #238, we had changed the type annotation from Decimal to our custom type FLOAT_DECIMAL https://github.com/open-contracting/credere-backend/pull/238/commits/56ea76a2ff1919637458d5f4b8e5e017eaf1856a, based on this documentation: https://docs.pydantic.dev/latest/api/standard_library_types/#decimaldecimal

Relevant comments:

• 238 issue description

• https://github.com/open-contracting/credere-backend/pull/238#issuecomment-1854577223
• https://github.com/open-contracting/credere-backend/pull/238#issuecomment-1854609403