Add ability to unassign a donation from a registration

Assigning a donation to a registration was one-way: the refund flow
could reverse the money but left the donation's "leg" payment in
place, permanently locking that portion of the donation as used even
though it had been refunded back out. Adds POST
/api/payments/unassign-donation, which deletes the leg, reverts the
registration's status/tickets the same way a refund downgrade already
does, and notifies the registrant. New "Assigned donations" list on
the supervisor Payments page surfaces existing legs with an Unassign
action, since no such list existed before.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/routes/paymentRoutes.js

@@ -8,6 +8,7 @@ const {
   getPaymentsByRegistration,
   getPaymentsByEvent,
   assignDonationToRegistration,
+  unassignDonationFromRegistration,
   createYocoCheckout,
   sendPaymentLink,
   createRefund,
@@ -27,6 +28,7 @@ router.get('/registration/:registrationId', protect, getPaymentsByRegistration);
 router.get('/', protect, supervisor, getPayments);
 router.get('/event/:eventId', protect, staff, getPaymentsByEvent);
 router.put('/assign-donation', protect, supervisor, assignDonationToRegistration);
+router.post('/unassign-donation', protect, supervisor, unassignDonationFromRegistration);
 router.post('/refund', protect, supervisor, createRefund);
 router.get('/admin/stats', protect, admin, getPaymentStats);