Populate payment method filter from real data instead of a guessed list

The previous fix hardcoded apple_pay/google_pay as extra filter options,
but Payment.method is free-text set by whatever the gateway reports, so
guessing at literal values was fragile and still didn't surface them for
this user. Add GET /api/payments/mypayments/methods returning the
distinct method values actually present in the user's payments, and have
the dashboard filter build its options from that instead. Also switch
the method filter from a startsWith match to an exact match, since the
values now come straight from the same column being filtered.

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

backend/src/routes/paymentRoutes.js

@@ -4,6 +4,7 @@ const {
   createPayment,
   getPayments,
   getUserPayments,
+  getUserPaymentMethods,
   getPaymentById,
   getPaymentsByRegistration,
   getPaymentsByEvent,
@@ -20,6 +21,7 @@ router.post('/', protect, supervisor, createPayment);
 router.post('/yoco-checkout', protect, createYocoCheckout);
 router.post('/yoco-checkout/send', protect, supervisor, sendPaymentLink);
 router.get('/mypayments', protect, getUserPayments);
+router.get('/mypayments/methods', protect, getUserPaymentMethods);
 router.get('/:id', protect, getPaymentById);
 router.get('/registration/:registrationId', protect, getPaymentsByRegistration);