Add read DEFAULT_MAX_UPLOAD_SIZE from environment

[EHJ-52n]

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[gemini-code-assist]

Code Review

This pull request modifies geonode/settings.py to allow the DEFAULT_MAX_UPLOAD_SIZE to be configured via an environment variable. Feedback was provided to improve the readability of the default value using a calculation and to enhance robustness when handling empty environment variables.

[giohappy]

@EHJ-52n fine to me. You're aware that this variable is only used once, the first time GeoNode is initialised.

[EHJ-52n]

Yes, but why?

Should we state this fact somewhere? In the settings.py itself, or maybe add it to the .env.sample next to DEFAULT_MAX_PARALLEL_UPLOADS_PER_USER?

[giohappy]

Yes, but why?

The unusual behaviour is due to the fact that the upload limits are stored in the DB to allow their management from the Django Admin. Allowing a dynamic configuration from the environment could potentially lead to conflicting settings.

https://docs.geonode.org/en/5.0.x/admin/upload-size-limits/index.html

I agree that this should be clarified, either in the code or the docs, or both.
NOTE: We're transitioning to mkdocs documentation, stored directly inside the GeoNode repo. The migration is not complete yet, but new docs or enhancements should go there first.

[EHJ-52n]

It is already documented in the help topic „Upload Size Limits“, which is not anchored in the TOC and in the environment variables topic below GeoNode Settings → Settings, which is anchored in the TOC.

I added some comments to the settings.py and stashed all commits in this PR via force-push.

Do you want me to add the same content to the sample.env?