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/API_DOCUMENTATION.md

@@ -240,6 +240,7 @@ 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+ | Distinct `method` values 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 |