Add story categories and populations to Story and StoryIdea forms

[Copilot]

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

Port the WordPress story submission form fields to the Rails app's /stories/new and /story_ideas/new forms. The WordPress form captures story categories (25 types like "Advocacy", "Domestic Violence", "LGBTQIA+") and story populations (7 types like "Adults", "Children", "Self") via multi-select checkboxes.

How did you approach the change?

Database:

• Added story_populations JSON column to stories and story_ideas tables
• Created migration seeding 25 story categories under new StoryCategory type

Models:

• Story: Added nested attributes for categorizable_items
• StoryIdea: Added categorizable_items associations and nested attributes
• Category: Added story_categories scope filtering by type

Controllers:

• Updated strong params to permit story_populations[], category_ids[], categorizable_items_attributes
• Load story categories in form setup methods

Views:

• Added category checkboxes (multi-select, 3-column grid)
• Added population checkboxes (multi-select, 4-column grid)
• Disabled fields respect promoted story state in StoryIdea form

Example checkbox implementation:

<%= f.collection_check_boxes :category_ids, @categories, :id, :name do |b| %>
  <div class="flex items-center">
    <%= b.check_box class: "rounded border-gray-300 text-purple-600" %>
    <%= b.label class: "ml-2 text-sm text-gray-700" %>
  </div>
<% end %>

Anything else to add?

Story populations use JSON arrays for flexible querying. Categories leverage existing polymorphic categorizable_items system used by workshops and other models.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add story_categories and story_populations to StoryIdea and Story</issue_title>
<issue_description>Figure out if these can become Sectors or if they need to be Categories or an entirely new model

Currently serviced via wordpress: https://stories.awbw.org/share/ (pics below).

We have most of this form under /story_ideas/new, and now need to add the multiselect checkboxes to that form.

Let's add story_populations as a json field to Story and StoryIdea

Both models will need to:
has_many :categorizable_items, dependent: :destroy, as: :categorizable
has_many :categories, through: :categorizable_items

And then the accepts_nested_attributes so these can pass via strong params
</issue_description>

<agent_instructions>follow the instructions in the description and reference the images and links to adapt the current system to be able to simulate under stories/new and story_ideas/new the same thing they have on wordpress currently.

</agent_instructions>

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

@maebeale

</comment_new>
<comment_new>@maebeale
cc @johnpaulashenfelter </comment_new>
<comment_new>@maebeale
@copilot follow the instructions in the description and reference the images and links to adapt the current system to be able to simulate under stories/new and story_ideas/new the same thing they have on wordpress currently.</comment_new>

• Fixes Add story_categories and story_populations to StoryIdea and Story #191

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