Fallback to crop specific photos in PhotoCapable thumbnail_url

[CloCkWeRX]

This pull request restores the crop photo fallback behavior for plantings, seeds, and harvests. When an item has no default photo of its own, its thumbnail_url now falls back to its associated crop's thumbnail_url rather than rendering a placeholder.

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PR created automatically by Jules for task 4973772339113599818 started by @CloCkWeRX

[google-labs-jules]

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[CloCkWeRX]

@jules remove the excess changes.

[google-labs-jules]

@jules remove the excess changes.

I have removed all of the environment, helper, and configuration files that were added to ease running tests locally. Only the core fallback implementation in app/models/concerns/photo_capable.rb, the photo association fix in app/models/photo_association.rb, and the unit tests in spec/models/planting_spec.rb remain.