Transferred-in reg still appears under the roster "unpaid" payment filter

Follow-up from #1944 / PR #2044.

A transferred-in registration's balance lives on its source registration, so it owes nothing to the new event. Reporting totals already exclude it (dashboard billable basis + EventRevenueFigures), but the roster payment-status filter does not: EventRegistration.not_paid_in_full (raw SQL summing allocations vs events.cost_cents) still matches a transferred-in reg (0 allocations, cost > 0), so it shows up under payment_status: "unpaid".

Suggested fix

• Exclude transferred-in regs from not_paid_in_full (and treat them consistently in paid_in_full / the payment_status scope), e.g. AND transferred_from_registration_id IS NULL, mirroring not_transferred_in.
• Confirm the roster "Paid/Unpaid" counts and the payment_status filter agree with the dashboard's billable basis.

Context

• app/models/event_registration.rb — paid_in_full / not_paid_in_full / payment_status scopes; not_transferred_in scope already exists.
• Deliberately scoped out of #1944 (a filter, not a reporting total).

Start in app/models/event_registration.rb by reading the paid_in_full, not_paid_in_full, payment_status, and existing not_transferred_in scopes. Compare their behavior with the dashboard billable basis and EventRevenueFigures, then check the roster Paid/Unpaid counts and payment_status filter. Done means transferred-in registrations are excluded consistently and the roster agrees with the dashboard totals.