Block CE/scholarship creation directly on a transferred-in registration

[maebeale]

Follow-up from #1944 / PR #2044.

A transferred-in registration's scholarship/CE/payments are meant to live on its source registration. The edit form hides the "Add scholarship" / "Add CE registration" cards for a transferred-in reg, but nothing stops an admin from reaching those new forms by URL and creating a scholarship or CE record against the transferred-in reg directly.

If that happens, the record sits on the wrong registration. Dashboard/revenue already exclude transferred-in regs from financial totals (billable basis), so it wouldn't inflate the new event's money — but it's still misfiled and would show nowhere useful.

Suggested fix

• Guard scholarship/CE creation (and allocation) when the target allocatable is a transferred-in EventRegistration — reject with a clear message pointing to the source reg, or redirect there.
• Consider the same guard for the bulk-payment allocation path (already excluded from the list, but the POST could still target one by id).

Context

• app/models/event_registration.rb — transferred_in?.
• ScholarshipsController#new/create, ContinuingEducationRegistrationsController#new/create, Events::BulkPaymentsController allocate path.
• Edit form already branches on transferred_in? to hide the cards (_form.html.erb / _transferred_in_financials.html.erb).