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/schemas.py

@@ -0,0 +1,85 @@
+"""
+支付模块 Pydantic 模型定义
+"""
+from pydantic import BaseModel
+from typing import Optional, Dict, Any
+
+
+class PaymentResult(BaseModel):
+    """创建支付订单的返回结果"""
+    success: bool
+    payment_method: str                          # wechat / alipay
+    out_trade_no: str                            # 内部订单号
+
+    # 微信支付参数（APP 调起支付所需）
+    wechat_params: Optional[Dict[str, str]] = None
+    # 支付宝 order_string（APP 调起支付所需）
+    alipay_order_string: Optional[str] = None
+
+    error_message: Optional[str] = None
+
+
+class NotifyResult(BaseModel):
+    """支付回调通知处理结果"""
+    success: bool
+    out_trade_no: Optional[str] = None           # 内部订单号
+    trade_no: Optional[str] = None               # 第三方交易号
+    total_amount: Optional[int] = None           # 金额（分）
+    trade_status: Optional[str] = None           # 交易状态
+    error_message: Optional[str] = None
+
+
+class PaymentStatus(BaseModel):
+    """查询支付状态的返回结果"""
+    success: bool
+    out_trade_no: str
+    trade_no: Optional[str] = None
+    trade_status: str                            # SUCCESS / NOTPAY / CLOSED / REFUND / PAYERROR
+    total_amount: Optional[int] = None
+    error_message: Optional[str] = None
+
+
+# === API 请求/响应模型 ===
+
+class CreateOrderRequest(BaseModel):
+    """创建订单请求"""
+    plan_id: str                                 # 套餐 ID
+    payment_method: str                          # wechat / alipay
+
+
+class CreateOrderResponse(BaseModel):
+    """创建订单响应"""
+    order_id: str                                # 内部订单号
+    payment_method: str                          # 支付方式
+
+    # 微信支付调起参数
+    wechat_params: Optional[Dict[str, str]] = None
+    # 支付宝 order string
+    alipay_order_string: Optional[str] = None
+
+
+class OrderStatusResponse(BaseModel):
+    """订单状态查询响应"""
+    order_id: str
+    plan_id: str
+    plan_name: str
+    amount: int                                  # 金额（分）
+    currency: str
+    payment_method: str
+    status: str                                  # pending / paid / failed / cancelled / refunded
+    trade_no: Optional[str] = None
+    created_at: str
+    paid_at: Optional[str] = None
+
+
+class OrderListResponse(BaseModel):
+    """订单列表项"""
+    id: str
+    plan_id: str
+    plan_name: str
+    amount: int
+    currency: str
+    status: str
+    payment_method: str
+    created_at: str
+    paid_at: Optional[str] = None