Fix video caching, moving to a synchronous upload to S3

[benoit74]

Rationale

Fix #82

Changes

• S3 upload is now synchronous with video encoding:
  • way simpler than the callback (which wasn't working anyway)
  • we expect video encoding to consume way more time than upload
  • we are already in parallel processing of videos to convert, so we do not mind much if the whole process takes long (we can just start more video processing threads if needed)
• removed the now useless pending_upload dictionary
• remove videos_futures dictionary by a simple list of futures
• cleanup the videos_futures list once future has completed
• add the callback to the video future asap (just in case it finishes early)

[codecov[bot]]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (cb214f4) 29.75% compared to head (d17109b) 29.97%. Report is 3 commits behind head on main.

Files	Patch %	Lines
src/kolibri2zim/scraper.py	31.57%	13 Missing :warning:
src/kolibri2zim/entrypoint.py	20.00%	4 Missing :warning:
src/kolibri2zim/processing.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ========================================== + Coverage 29.75% 29.97% +0.21% ========================================== Files 6 6 Lines 773 764 -9 Branches 135 137 +2 ========================================== - Hits 230 229 -1 + Misses 531 523 -8 Partials 12 12 ```

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