Set custom default value for content-header-bytes-length

[benoit74]

We have regular occurrences of zimit failures due to the charset encoding being too far at the beginning of the HTML document. By default, we expect it to be in the first 1024 bytes. This is a sensible default because recommendations are that this should be at the beginning of the HTML head. Not all sites follow this convention, and many are far from that.

I propose to override default content-header-bytes-length only on zimit-frontend to accommodate more sites for non-knowledgeable persons.

We do not want to expose this setting on zimit.kiwix.org because it is too complex.

We should not change warc2zim default (1024) which is still the best comprise value, when you have someone knowledgeable using the tool, because it avoid spending too much resources on this.

WDYT?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 45.99%. Comparing base (4a201ec) to head (d015a24).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
api/src/zimitfrontend/routes/requests.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
- Coverage   46.11%   45.99%   -0.12%     
==========================================
  Files          10       10              
  Lines         386      387       +1     
  Branches       44       44              
==========================================
  Hits          178      178              
- Misses        206      207       +1     
  Partials        2        2              

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