Hide the per-item credit preference field from non-admins once profiles are self-service

Interim state shipped in #2093.

Why the field is there now

Credits render from the credited person's profile (Person#display_name_preference plus anonymous_contributions). Facilitators cannot edit their own profile — PersonPolicy#edit? and #update? are admin? — so the submission forms still collect a per-item credit preference as a way to record what the submitter wants.

That stored value does not drive display. It surfaces on /author_credit_divergences as a discrepancy, and an admin applies it to the person's profile so it covers everything they share. The edit forms already warn when a record diverges, linking to that page.

What to do here

Once facilitators can edit their own display preferences:

• Hide author_credit_preference on the three submitter-facing idea forms (story_ideas, workshop_ideas, workshop_variation_ideas) unless allowed_to?(:manage?, ...) — the field lives in app/views/shared/_author_credit_status.html.erb
• Drop :author_credit_preference from those three controllers' strong params, or keep it admin-only
• Point submitters at their own profile instead — replace the "an admin applies it to your profile" copy in app/views/shared/_author_credit_preview.html.erb
• Revisit whether the preference section of the divergences page is still needed, or only the three sections about a missing author_id

Related

• Self-service profile editing is the blocker: PersonPolicy#edit? / #update? are admin-only, and person_params permits notes, filemaker_code, created_by_id and nested user_attributes[:inactive], so it needs its own narrow whitelist.
• The per-item anonymous override is a separate legacy path the divergences page also drains.

🤖 From Claude: filed while shipping the interim step in #2093.

Start by reading app/views/shared/_author_credit_status.html.erb and app/views/shared/_author_credit_preview.html.erb, then trace the three submitter-facing idea forms and their controllers. Confirm the self-service profile-editing prerequisite, restrict or remove the preference field and params, update the guidance, and determine whether the preference section remains on /author_credit_divergences.