Pass User-Agent to all web requests, set async job timeout, and retry header retrieval

[benoit74]

Fix #79

Changes:

• pass a User-Agent compliant with upload.wikimedia.org Policy in all web requests
• set a job timeout for all async jobs, serving as a "fallback" should something go terribly wrong (avoid scraper being stuck forever and never failing)
• move code fetching online header in the try...catch for proper operation of logic avoiding to retry some known bad assets

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 48.27586% with 15 lines in your changes missing coverage. Please review.

Project coverage is 43.48%. Comparing base (08e9733) to head (49bff38).

Files with missing lines	Patch %	Lines
scraper/src/mindtouch2zim/asset.py	12.50%	7 Missing :warning:
scraper/src/mindtouch2zim/download.py	68.75%	5 Missing :warning:
scraper/src/mindtouch2zim/processor.py	33.33%	2 Missing :warning:
scraper/src/mindtouch2zim/entrypoint.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #92 +/- ## ========================================== + Coverage 43.14% 43.48% +0.33% ========================================== Files 15 16 +1 Lines 978 998 +20 Branches 133 134 +1 ========================================== + Hits 422 434 +12 - Misses 545 553 +8 Partials 11 11 ```

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