Explicitly set BaseMixin id to autoincrement

[kmshin1397]

This is the implicit default behavior when id is a single-column primary key, but autoincrement=True must be set explicitly when the id column is used as a part of a multiple-column primary key. This allows subclassed models to specify additional columns as part of the primary key.

https://docs.sqlalchemy.org/en/14/core/metadata.html?highlight=autoincrement#sqlalchemy.schema.Column.params.autoincrement

[pep8speaks]

Hello @kmshin1397! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file app/models.py:

Line 954:80: E501 line too long (89 > 79 characters)