Enhance Database Modeling and SQLAlchemy Compatibility

[cubxxw]

This pull request makes several changes to the backend/app/models.py file to enhance database modeling and improve compatibility with SQLAlchemy. The key updates include adjustments to field definitions, changes to relationships, and the removal of certain constraints.

Changes to field definitions:

• Updated the email field in UserBase to explicitly use sa_type=String(255) for better SQLAlchemy compatibility.
• Changed the email field type in UserRegister, UserUpdate, and UserUpdateMe from EmailStr to str for consistency and to avoid type-related issues.

Changes to relationships:

• Modified the items relationship in the User model to include sa_relationship_kwargs for defining cascading behavior and specifying join conditions.
• Updated the owner_id field in the Item model to remove the foreign_key constraint while keeping it indexed for performance. Added sa_relationship_kwargs to the owner relationship for explicit join conditions.