Add attachment fles for lessons

[payamnj]

Overview

Allow course editors to attach files to lessons. Attached files are included as email attachments when the lesson is sent to learners.

Requirements

Backend

• Add a new LessonAttachment model with a ForeignKey to Lesson, storing the uploaded file
• A lesson can have multiple attachments, but the total size of all attachments must not exceed 15 MB
• Enforce the size limit at the model/API level and return a clear validation error when exceeded
• Update the send lesson command to include all lesson attachments as email attachments

Frontend

• Update the lesson form to allow uploading and managing attachment files (add, remove)
• Display existing attachments when editing a lesson
• Show the total attachment size and enforce the 15 MB limit with a user-friendly error message

Acceptance criteria

• LessonAttachment model exists with ForeignKey to Lesson and a file field
• Total attachment size per lesson is validated and capped at 15 MB
• Attachments are included in the lesson email when sent
• Lesson form UI supports adding and removing attachments

[payamnj]

We can use a shared model for linking attachments and uploaded images to a lesson.