Add organization management and admin-specific APIs

- Introduced schemas for organizations, including creation, updates, and responses.
- Created models for `Organization` and `UserOrganization` with role-based access control and relationships.
- Implemented admin APIs for managing users, organizations, and bulk actions.
- Added advanced filtering, sorting, and pagination for user and organization queries.
- Updated `CRUD` logic to support organization-specific operations and member management.
- Enhanced database with necessary indexes and validation for improved performance and data integrity.

backend/app/models/__init__.py

@@ -9,8 +9,11 @@ from .base import TimestampMixin, UUIDMixin
 # Import models
 from .user import User
 from .user_session import UserSession
+from .organization import Organization
+from .user_organization import UserOrganization, OrganizationRole
 
 __all__ = [
     'Base', 'TimestampMixin', 'UUIDMixin',
     'User', 'UserSession',
+    'Organization', 'UserOrganization', 'OrganizationRole',
 ]