Almost everything done.
It seemed like more than 3 hours but ok.
I couldn’t solve the problem with slow segment downloading with yt-dlp. So, I left it how it was. Yes it is slow, yet it works. From the backend part what I added was
Format selection, Now the resolution of the video can be selected.
Attaching download to the user_id small change, yet now download logs are saved when user login.

My goal was not learning frontend, so this part I gave it to AI. I used Codex skill and made pretty good frontend.

But again, even if frontend was done by AI, the backend was done completely by me. So, it still required huge effort.