Remove legacy age_range fields, add migration rake task

[maebeale]

Summary

• Remove age_range / age_range_spanish from workshop display (show page, forms), strong params, decorators, model validations, and specs
• Remove rhino_age_range_spanish rich text field from Workshop and WorkshopIdea models
• Remove age_range from WorkshopIdea entirely (model, controller, service, specs)
• Add data:convert_age_ranges rake task to migrate free-text age_range values into AgeRange categorizable_items
• Rake task creates [AGE_RANGE_DATA] comments on workshops for audit trail (auto-applied and unmatched)
• Nils column values that exactly match a category name
• Rename "Teen" seed to "13-17"

Dependencies

• PR 1 (prerequisite): Workshop commentable (has_many :comments) — needed before running the rake task in production

Test plan

• Verify workshop show page no longer displays age range fields
• Verify workshop edit form no longer has age_range/age_range_spanish inputs
• Verify workshop idea creation no longer accepts age_range
• Run bundle exec rspec spec/tasks/convert_age_ranges_spec.rb (21 examples, 0 failures)
• Run rake task against staging data and verify categorizable_items and comments are created

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