[WIP] Add CRUD functionality for WorkshopVariationIdea

[Copilot]

Plan: Add WorkshopVariationIdea CRUD with Promote to WorkshopVariation

Following the same pattern as StoryIdea→Story and WorkshopIdea→Workshop:

• Create database migration for workshop_variation_ideas table
• Create WorkshopVariationIdea model with associations
• Create WorkshopVariationIdeasController with CRUD actions
• Create view files for WorkshopVariationIdea (index, show, new, edit, _form)
• Add routes for workshop_variation_ideas resource
• Create WorkshopVariationIdeaDecorator
• Create WorkshopVariationFromIdeaService to handle promotion
• Update WorkshopVariationsController to handle promotion from ideas
• Add "Promote to WorkshopVariation" button in edit form
• Add factory for testing
• Add specs for model, controller, and routing
• Run migrations and test the feature manually
• Run code review and security checks

Original prompt

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This section details on the original issue you should resolve

<issue_title>Do you audit/edit WorkshopVariations or just publish them?</issue_title>
<issue_description>- Curious if we should set up WorkshopVariationIdea so it follows the same process as StoryIdea and WorkshopIdea</issue_description>

<agent_instructions>create a new pr that adds WorkshopVariationIdea CRUD, and on the edit form has a Promote to WorkshopVariation button, just like with WorkshopIdea->Workshop and StoryIdea->Story</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Do you audit/edit WorkshopVariations or just publish them? #422

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