Refactor download.py

[deldesir]

This pull request refactors the download.py file to improve code readability, maintainability, and performance.

Changes:

• Refactored code to use SQLAlchemy for database interactions.

• Use of xb_utils.py to query the xklb-metadata.db database, enhancing the efficiency of metadata extraction and processing.

• Tested on Ubuntu 24.04

[holta]

How was this tested?

Thanks @deldesir!

[deldesir]

How to test

• Pull the PR following How do I test a pull request (PR) ? instructions.
• Submit a YouTube video URL via the “Download to IIAB” button and monitor the “Task” page to check whether downloading is reported or not.
• On success, verify with datasette to review the extraction with datasette /library/calibre-web/xklb-metadata.db -h 0.0.0.0 -p <PORT_NUMBER> and then point my browser to http:<IIAB_URL>/books:<PORT_NUMBER>. If metadata are extended and the media.path is actually a path instead of a URL for the required video, that means the refactoring was effective and functionality is preserved.