fix: resolve AttributeError in User model and ensure consistent password handling

- Fixed the `AttributeError: 'User' object has no attribute '_password'` by properly mapping the `_password` attribute to the `password` column in the database.
- Updated the `User` model to ensure passwords are only hashed once during creation and not re-hashed when retrieved or updated.
- Improved the `check_password` method to correctly compare hashed passwords.
- Verified the signup and login flow to ensure consistent behavior

server.py

@@ -10,6 +10,7 @@ load_dotenv()
 app = Flask(__name__)
 CORS(app)
 
+app.secret_key = os.getenv('SECRET_KEY')
 app.config['SQLALCHEMY_DATABASE_URI'] = os.getenv('DATABASE')
 app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False