fix(payment): critical audit fixes for security, idempotency and correctness

Backend fixes:
- #1: doSub subscription idempotency via audit log check
- #2: markFailed only when status=RECHARGING (prevents overwriting COMPLETED)
- #3: ExpireTimedOutOrders checks upstream payment before expiring
- #4: Public verify endpoint for payment result page (no auth required)
- #5: EasyPay QueryOrder returns amount, confirmPayment handles zero amount
- #6: WxPay notifyUrl priority: request-first, config-fallback
- #7: EasyPay remove double URL decode in VerifyNotification
- #8: checkPaid/cancelUpstreamPayment use order's provider instance
- #9: Amount NaN/Inf/negative validation in order creation and refund
- #10: Refund amount comparison uses tolerance instead of float64 ==
- #11: Skip balance deduction on retry when previous rollback failed
- #12: checkPaid logs fulfillment errors instead of silently ignoring
- #13: WxPay certSerial added to required config fields

Frontend fixes:
- Payment result page no longer requires authentication
- Public verify API fallback for expired sessions

backend/internal/payment/provider/wxpay_test.go

@@ -156,6 +156,7 @@ func TestNewWxpay(t *testing.T) {
 		"apiV3Key":    "12345678901234567890123456789012", // exactly 32 bytes
 		"publicKey":   "fake-public-key",
 		"publicKeyId": "key-id-001",
+		"certSerial":  "SERIAL001",
 	}
 
 	// helper to clone and override config fields