HOLD: Hide scholarship/CE registration boxes when they don't apply

[maebeale]

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

• The registration edit form's Organizations / Scholarship / CE credits row showed the Scholarship and CE boxes unconditionally.
• Scholarship is only meaningful when there's a fee to subsidize, so it shouldn't appear on free events.
• Continuing-education credits aren't offered for every event, so admins need a way to flag eligibility — and the CE box should hide when an event doesn't grant credits.

How did you approach the change?

• Added ce_credits_eligible to Event (boolean, default true, not null) via migration; permitted it in EventPolicy and surfaced a checkbox under Event Cost on the event form.
• On the registration form (event_registrations/_form):
  • Hide the Scholarship box when the event is free (cost_cents == 0).
  • Hide the CE credits box when event.ce_credits_eligible? is false.
  • The Organizations box absorbs the freed columns in the 4-col grid:
    • paid + CE-eligible → orgs at sm:col-span-2 (both boxes show)
    • one box hidden → orgs at sm:col-span-3
    • free + not CE-eligible → orgs at sm:col-span-4 (full width)

UI Testing Checklist

• Event form shows "Eligible for continuing education credits" (checked by default)
• Paid + CE-eligible registration: Organizations (half), Scholarship, CE credits all show
• Free event: Scholarship box hidden, Organizations widens
• Event with CE eligibility unchecked: CE box hidden, Organizations widens
• Free + not CE-eligible: only Organizations, full width

Anything else to add?

• 🚧 HOLD — opening as a draft for review of the approach before marking ready.
• Open question: the request mentioned the ticket and registrations index displays too. Those currently render no Scholarship/CE/Organizations boxes (ticket shows cost/payment only; index is a plain table), so there was nothing to gate there. Happy to add CE/scholarship surfacing to those if that's the intent — let me know the desired design.
• Tests: model default, 4 system specs covering each visibility/width combination, plus existing edit-form specs (use the default paid + CE-eligible event) still pass.

🤖 Generated with Claude Code

[maebeale]

🤖 From Claude: The span is mapped to literal class strings (sm:col-span-2/3/4) rather than interpolated (sm:col-span-#{org_span}) so Tailwind's content scanner actually compiles them — interpolated class names wouldn't be generated.