Fix Bugs in OnDemand Course Functionality (Pre-Purchase Experience)

This task aims to resolve three user-facing bugs encountered in the OnDemand course experience before a course has been purchased. These issues impact usability and the perception of platform reliability.

1. Progress Tracking for Chapter 2 Before Purchase

Description:
Users are seeing their progress being tracked for Chapter 2 of a course even before purchasing the course. This should not occur, as tracking should begin only after a successful course purchase.

Expected Behavior:
Progress tracking should be disabled for all chapters until the user has purchased the course. Chapter 2 should behave the same as all other locked content.

https://sharing.clickup.com/clip/p/t9004002740/52c984b0-c5fb-409c-ab54-21d25141f237/screen-recording-2025-05-30-14%3A50.webm

2. Lecture Thumbnails Not Shown Before Purchase

Description:
Lecture thumbnails uploaded are not displayed in the course preview prior to purchase. The backend is currently returning null for these assets, resulting in broken or missing visuals in the UI.

Expected Behavior:
Each lecture should show a thumbnail, even before purchase, unless not changed from user in course builder.

3. Lecture Duration Not updated after re-publish course

Description:
Lectures updated in the course preview are displaying a duration referred to the original lecture uploaded, regardless of the actual length of the content.

Expected Behavior:
Lecture durations should reflect the real total time (e.g., 5:30, 12:47) based on the video actually uploaded.