Migrate Values submodule to SQLAlchemy

[takakonishimura]

We currently query SQLite directly with (raw query strings)[https://github.com/Project-Reclass/toynet-flask/blob/main/flasksrc/value.py#L66]. This is error prone and also must be changed every time we switch to a new relational database. (SQLAlchemy)[https://www.sqlalchemy.org/] is a popular and robust solution to this problem.

This ticket explores how to implement SQLAlchemy with our most basic submodule, Values:

• API registration and API implementation
• db schema, seed data, and seeding call
• unit tests

There should not be many changes to the database schema, and all tests should pass the same.

Don't forget to update requirements.txt!