fix(tasks): pre-generate thumbnails for active_storage

[tvdeyen]

What is this pull request for?

The alchemy:generate:thumbnails rake task was written for Dragonfly: it called PictureThumb.generate_thumbs! directly and warmed ingredient variants by requesting their urls — both of which only persist a thumbnail as a Dragonfly side effect. Under the now-default active_storage adapter it generated nothing, and its :thumbs preload references a Dragonfly-only association that does not exist there, so the ingredient task broke outright.

The logic now lives in an Alchemy::GenerateThumbnails service that branches on the configured storage adapter and materializes each variant, so existing pictures and their ingredient crop and srcset variants are warmed on both adapters. The service is pure and reports progress through a block, so the rake task can log progress without the service depending on the shell.

Notable changes

• Background generation (ASYNC=true). Warming every thumbnail inline can take a long time on large sites. Passing ASYNC=true to the tasks enqueues the active_storage variants as native ActiveStorage::TransformJobs instead of processing them inline. Dragonfly has no background processor, so ASYNC is a no-op there and generation runs inline.
• srcset fix. The old task merged a size string into the render options hash, which raises TypeError; it was never hit because no element configured srcset. It now builds the variant the same way the frontend does (picture_url(size: ...)), so the pre-generated variant matches what the browser requests.

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 98.27%. Comparing base (e9e13f8) to head (3fc2622).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4037   +/-   ##
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  Coverage   98.26%   98.27%           
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  Files         350      351    +1     
  Lines        9184     9219   +35     
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+ Hits         9025     9060   +35     
  Misses        159      159           

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[tvdeyen]

Let's use ActiveJob and add a async argument to the rake task, so if needed we can background the thumbnail generation