Add tags section with categories and sectors to story ideas form

[Copilot]

What is the goal of this PR and why is this important?

Story ideas now support tagging with categories and sectors, matching the functionality available in the stories form. This enables better organization and filtering of story ideas before they're promoted to full stories.

How did you approach the change?

Model layer:

• Added sectorable_items association and sectors through-association to StoryIdea

Controller layer:

• Set up @categories_grouped and @sectors in set_form_variables
• Added assign_categories private method to assign tags before save
• Updated strong params to permit category_ids and sector_ids arrays
• Removed legacy categorizable_items_attributes parameter

View layer:

• Added collapsible Tags accordion with Sectors and Categories sections
• Sectors displayed with checkboxes for all published sectors
• Categories grouped by type (Age Range, Story Category, etc.) with checkboxes
• Hidden fields ensure empty arrays submit when no tags selected
• Tags disabled when story promoted to story

Implementation mirrors the stories form pattern for consistency.

Anything else to add?

The assign logic runs before save to avoid redundant database operations. Strong parameters include category_ids and sector_ids for Rails security, though assign_categories accesses params directly to match the existing stories controller pattern.

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