I have just created this simple python flask auth system for learning purpose so that you can implement it in flutter as well as in web for learning purpose.
pip install flask
pip install SQLAlchemy
replace username email@provider.com and password with the actual values
CURL -X POST http://127.0.0.1:5000/register -d "uname=username&mail=email@provider.com&passw=password"
if Status is equal to 11 it mean successfully created account if 22 it mean username already exist
replace username with email@provider.com and passw with password
CURL -X POST http://127.0.0.1:5000/login -d "uname=username&passw=password"
if Status is equal to 11 it mean successfully login if 22 it mean username or password is incorrect