Allow allocating bulk payments to CE registrations

🤖 From Claude:

Follow-up from #2072 (CE payments reporting).

Context: Bulk payments can currently only be allocated to EventRegistration records — Events::BulkPaymentsController#allocate looks up an EventRegistration and builds the allocation against it, and allocated_cents_by_registration is scoped to allocatable_type: "EventRegistration". CE fees are billed on ContinuingEducationRegistration and flow through the same polymorphic allocation system, so a bulk payment cannot presently be applied to a registrant's CE balance.

Ask: Let an admin allocate part of a bulk payment to a registrant's CE registration, not just their event registration. This needs UI work on the bulk-payments allocation card (a way to pick the CE registration as the allocation target) plus backend support in allocate.

Notes:

• The "already allocated" figure shown per registration in the bulk-payments UI is intentionally registration-fee-scoped and should stay that way; CE allocations are tracked separately.
• Deferred intentionally in #2072 because it isn't a small change (the allocate form only knows event registrations today).

Start with Events::BulkPaymentsController#allocate and the bulk-payments allocation card, then read issue #2072 for the existing payment-reporting context. Trace allocated_cents_by_registration and the polymorphic allocation records to understand the current EventRegistration path. Done means an admin can select a ContinuingEducationRegistration, allocate part of a bulk payment to it, and retain the existing registration-fee allocation display.