Replace WYSIWYG editor with Markdown editor across all relevant models and admin fields. Add utilities for rendering and stripping markdown. Adjust serializers, views, and templates to support markdown content. Introduce `PastedImage` model and upload endpoint for handling inline image uploads in markdown.
This change simplifies content formatting while enhancing flexibility with markdown support.
Fixes: 1) Correct default values and type handling in util constructors; 2) Resolve missing or ambiguous `cast` operations for dynamic typing in tests and views; 3) Address potential issues with fallback/default handling in middleware.
Extra: 1) Refactor test cases to ensure stricter adherence to typing hints and valid contracts; 2) Update docstrings to align with new type annotations; 3) Cleanup unused imports and add comments for improved maintainability.
