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/README.md

@@ -260,6 +260,7 @@ 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+ | Distinct `method` values 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 |