Auto-generate and update model schema

As a developer, 
I want to automatically generate and update the model schema, 
similar to the implementation in the card-api project, 
so that any model changes are reflected in an updated schema diagram in the README.md file.

Background

Currently, there is no automated process to generate and maintain an updated model schema diagram for the activate-account/api. In the card-api project, this was achieved using django-extensions and pydot, which allow generating a model diagram using a command.

To maintain documentation accuracy, the system should detect model changes and regenerate the diagram automatically, ensuring the latest version is added to the README.md file.

Acceptance Criteria

• Implement auto-generation of the model schema diagram for activate-account/api using django-extensions and pydot.
• Ensure that any model changes trigger an update to the diagram.
• Update the documentation (README.md) with the latest model schema diagram.