Cache subtitles in S3 storage

[dan-niles]

This PR modifies the download_subtitles method to cache subtitles in S3. The modified method now works as follows:

• Fetch requested subtitle keys using yt-dlp (e.g., en, fr, de) and store them in requested_subtitle_keys.
• Iterate through the requested_subtitle_keys and attempt to download each subtitle file from the S3 cache. If a file is - successfully downloaded from the S3 cache, remove the corresponding key from requested_subtitle_keys.
• For the remaining keys in requested_subtitle_keys, download the subtitles using yt-dlp.
• Upload the newly downloaded subtitles to the S3 cache.
• Save the information about the fetched subtitles in a local cache as a JSON file for future use.
• Add the downloaded subtitles to the ZIM file.

Close #277

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 34 lines in your changes missing coverage. Please review.

Project coverage is 1.50%. Comparing base (60b85b5) to head (d34a734).

Files	Patch %	Lines
scraper/src/youtube2zim/scraper.py	0.00%	34 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #287      +/-   ##
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- Coverage   1.54%   1.50%   -0.05%     
========================================
  Files         11      11              
  Lines       1102    1132      +30     
  Branches     162     170       +8     
========================================
  Hits          17      17              
- Misses      1085    1115      +30     

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[dan-niles]

Original goal I expressed in the issue was to avoid being blocked by yt-dlp ban when subtitles did not changed.

The reason I used yt-dlp to get the list of subtitles is that the scraper doesn't know which language subtitles need to be downloaded. Without the --all-subtitles flag, only the default available subtitles for the video are downloaded. With the --all-subtitles flag, auto-generated subtitles are included as well.

To avoid calling yt-dlp entirely in this scenario, we could save two zipped files in the S3 cache:

1. subtitles/{video_id}/default.zip - containing only the default subtitles.
2. subtitles/{video_id}/all.zip - containing all subtitles when --all-subtitles is passed.

WDYT?

[benoit74]

This seems a potential improvement, but does it really help the scraper run faster? Because it has the drawback that we do not know when we should invalidate these to update them.

[benoit74]

Let's pause this issue/PR to let me reflect a bit on this