Include future-scheduled elements when publishing pages

[tvdeyen]

What is this pull request for?

Previously, only currently visible elements were copied to the public version when publishing a page. This meant elements scheduled for future publication required a republish once their public_on date arrived.

Now the publisher copies all publishable elements, which includes both currently visible elements and those scheduled for the future. This allows elements with a future public_on date to automatically become visible once their scheduled time passes without requiring a republish.

Notable changes

• Add publishable? method to Publishable concern
• Add publishable filter to ElementsRepository
• Update Publisher to use publishable instead of visible

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.44%. Comparing base (0f0872b) to head (c395e7a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3592   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files         314      314           
  Lines        8190     8194    +4     
=======================================
+ Hits         7981     7985    +4     
  Misses        209      209           

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