Transferred-in ticket: surface source financials and service them from the new registration

#2044 合并后，从 EventRegistration 开始，检查现有的 transferred_in? delegation methods 和 source financials partial。然后跟踪 app/views/events/callouts/payment.html.erb、Events::CalloutsController、Events::RegistrationsController、event_invoice.rb、event_receipt.rb、builtin_callout_cards.rb 和 pay_charge_extensions.rb。为 source-payment delegation、scholarship servicing、payment-page rendering 和 pay routing 添加 model specs 和 request specs，然后运行 ai/test_extra；完成的标准是 source-held payments 和 scholarships 从新的 ticket 中得到处理，且不会错误归档 payments。

Followup to the event-registration transfer work (#1944 / PR #2044). Start as a new branch off main after #2044 merges — not part of #2044.

Context

A transferred-in registration's registrant-facing ticket/callout pages don't correctly surface or service the money/scholarship that live on the source reg. The read-side "amount due" already delegates to the source; the gaps are the payment ledger, scholarship servicing, and a pay-flow that misfiles a payment onto the transferred-in reg without clearing the delegated balance.

Caveats (main moved since this was drafted):

• Line numbers below are approximate — main advanced through #2237/#1993/#2178/#2258, so verify against current code (method names are stable).
• #2178 (scholarship decline flow) is now merged and reworked the scholarship agreement into a tri-state agreement_response_status (pending/accepted/declined) + ScholarshipAgreementResponse history. Part 2 (scholarship servicing) must be built against that new model, not the old agreement_signed_at boolean.

The governing rule

Delegation to the source is conditional on the source actually having allocations/payments.

• Source has payments → the financial record is anchored on the source. Show the source's ledger + delegated balance on the new ticket; to pay a remaining balance, send the registrant to the source reg's ticket (pay there, allocations stay on source). Never let a payment land on the transferred-in reg in this case.
• Source has no payments → the math should all be on the new reg. The new reg owns its own cost/balance/allocations; the registrant pays on the new reg normally (no delegation). This also removes the misfiling risk.

Implementation seam: gate the existing transferred_in? delegation in EventRegistration (remaining_cost, paid_in_full?, payment_received?, payment_access_granted?, invoice_available?, etc.) on transferred_from_registration.allocations.exists? (or payments_sum.positive?). Add a predicate like source_holds_financials? = transferred_in? && source.allocations.exists? and key the delegations + the pay routing off it. Verify the invoice/receipt from_registration swap (app/presenters/event_invoice.rb, event_receipt.rb) respects the same condition.

Part 1 — Payment ledger

app/views/events/callouts/payment.html.erb (+ payment action, Events::CalloutsController):

• Keep this reg's own allocations rendering as-is (the existing @allocations list).
• If transferred-in AND the source has allocations, add a separate section below — "Payments on your original registration" — rendering source.allocations. Reuse the figures the admin summary already computes in app/views/event_registrations/_transferred_in_financials.html.erb (source.event.cost_cents, source.allocations_sum/payments_sum, due).
• "Amount due" already correct via delegated remaining_cost — leave it, but it now follows the conditional-delegation rule.

Part 2 — Scholarship servicing

Let the registrant view + respond to the source's scholarship from the new ticket, showing the old reg's scholarship record.

• scholarship action gate + @scholarship: resolve via the chain — EventRegistration#effective_scholarship (scholarships.first || transferred_from_registration&.effective_scholarship) instead of raw @event_registration.scholarships.first. Relax the gate so a transferred-in reg whose source has a scholarship isn't redirected away.
• sign_agreement (and the new accept/decline flow from #2178): operate on the source's Scholarship record (same person).
• scholarship_status_card builder (app/services/builtin_callout_cards.rb): use the effective scholarship for the card's gate + data.
• Task completion stays admin-only (display only).
• The scholarship view's admin "Edit scholarship" link should point at the source's scholarship record.

Part 3 — Pay-flow correctness

• pay action + redirect_to_stripe_checkout (Events::RegistrationsController) and the webhook allocation (pay_charge_extensions.rb): when source_holds_financials?, route the checkout/allocation to the source reg (metadata event_registration_id = source), so paying clears the delegated balance. When the source holds no financials, the transferred-in reg pays itself (correct once remaining_cost no longer delegates in that case).
• On the ticket/payment page, the pay button for a transferred-in reg with a source balance should link to the source's payment/ticket rather than posting to its own pay path.

Per-builtin (no change needed)

• ce_hours — correct (hours ride on the new reg's own record).
• certificate — correct (earned at the intended/new event).
• videoconference / staff / handouts / faq — correct (this event's content); keep showing (they're attending here), unlike a transferred-out reg.
• art_supplies — reserved key, unused; ignore.

Verification

• Model specs for conditional delegation: transferred-in with source payments → delegates; with no source payments → own math.
• Request specs: payment page shows the source-allocations section only when transferred-in + source has allocations; scholarship page renders + services the source's scholarship from the new slug; pay routing targets the correct reg.
• ai/test_extra before opening the (draft) PR.

🤖 Filed by Claude for maebeale — followup to #1944.