Switch payment method filter to a static list with an "other" bucket

Per feedback, drop the dynamic /mypayments/methods lookup (wasn't
loading reliably) in favor of a static cash/card/eft/voucher/other
dropdown. Server-side normalization now folds any gateway-reported
method outside those four manual-entry values (apple_pay, google_pay,
yoco, etc.) into "other" instead of "card", both in the returned data
and in the filter query.

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

backend/API_DOCUMENTATION.md

@@ -240,7 +240,6 @@ For unauthenticated (guest) registration on events with `requiresAuth: false`:
 | POST | `/api/payments` | supervisor+ | Record manual payment (cash/EFT) |
 | POST | `/api/payments/yoco-checkout` | user+ | Initiate Yoco checkout; re-evaluates early-bird pricing first — returns `{ priceUpdated: true, newTotal }` if prices changed |
 | GET | `/api/payments/mypayments` | user+ | Own payments |
-| GET | `/api/payments/mypayments/methods` | user+ | Normalized method options (cash/card/eft/voucher) present in the user's own payments |
 | GET | `/api/payments/:id` | user+ | Payment by ID |
 | GET | `/api/payments/registration/:registrationId` | user+ | Payments for a registration |
 | GET | `/api/payments` | supervisor+ | All payments |