feat: 新增后端支付模块，支持微信和支付宝

- 新增api/payment/支付服务（微信、支付宝）
- 新增api/routers/payment.py支付路由
- 更新database/models.py支付相关模型
- 新增数据库迁移文件（订单表、用户订阅字段）
- 更新main.py、requirements.txt、.env.production

Co-authored-by: Cursor <cursoragent@cursor.com>

api/payment/__init__.py

@@ -0,0 +1,23 @@
+"""
+独立可复用的支付模块
+支持微信支付 APP 支付和支付宝 APP 支付
+"""
+from .service import PaymentService
+from .config import PaymentConfig
+from .exceptions import (
+    PaymentError,
+    PaymentConfigError,
+    PaymentCreateError,
+    PaymentNotifyError,
+    PaymentQueryError,
+)
+
+__all__ = [
+    "PaymentService",
+    "PaymentConfig",
+    "PaymentError",
+    "PaymentConfigError",
+    "PaymentCreateError",
+    "PaymentNotifyError",
+    "PaymentQueryError",
+]