refactor: replace direct WeChat/Alipay with unified pay-api gateway

Switch from direct WeChat Pay / Alipay integrations to the unified
宇之然 pay-api gateway (HMAC-SHA256 auth). Removes wechat_pay.py,
keeps PaymentGateway abstraction, adds UnifiedPayService. Simplifies
payment.py create_order to {plan, pay_type} params. Single webhook
endpoint replaces separate WeChat/Alipay notify handlers.

backend/pytest.ini

@@ -4,6 +4,7 @@ python_files = test_*.py
 python_classes = Test*
 python_functions = test_*
 addopts = -v --tb=short --cov=app --cov-report=term-missing
+asyncio_mode = auto
 filterwarnings =
     ignore::DeprecationWarning
     ignore::PendingDeprecationWarning