fix concurrency issue: use scoped session

app/database.py

 from sqlalchemy import create_engine
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker
+from sqlalchemy.orm import scoped_session
 
 SQLALCHEMY_DATABASE_URL = "sqlite:///./sql_app.db"
 
-engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False})
-SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}) # echo=True
+SessionLocal = scoped_session(sessionmaker(autocommit=False, autoflush=False, bind=engine))
 Base = declarative_base()
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