This commit refactors the LMS architecture from a flat 2-tier structure to a granular 3-tier structure to support better course organization, while ensuring zero downtime or breaking changes for existing frontend clients.
Introduced `CourseChapter` model as the new grouping layer
2 endpoints (`/v2/.../lessons/`): Introduced new `LessonListV2APIView` and `CourseChapterSerializer` to return the new nested JSON schema (Chapters -> Lessons) for the upcoming accordion UI.
Created robust `migrate_lessons_to_chapters` management command.
Registered `CourseChapter` and added `CourseChapterInline` to `CourseAdmin` for seamless content management.
Mohsen Taba