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/README.md

@@ -260,7 +260,6 @@ Full interactive API docs available at `GET /docs` (requires admin JWT — pass
 | POST | `/` | supervisor+ | Record manual payment |
 | POST | `/yoco-checkout` | user+ | Initiate Yoco checkout |
 | GET | `/mypayments` | user+ | Own payments |
-| GET | `/mypayments/methods` | user+ | Normalized method options (cash/card/eft/voucher) present in the user's own payments |
 | GET | `/:id` | user+ | Payment by ID |
 | GET | `/registration/:registrationId` | user+ | Payments for a registration |
 | GET | `/` | supervisor+ | All payments |