📝 docs(prd): add payment refund flow sequence diagram

- 添加支付退款流程完整时序图，包含支付流程、支付回调、退款流程和退款回调四个阶段
- 使用mermaid语法绘制，明确用户、Mini小程序、订单API、支付API、微信支付和退款API之间的交互流程

docs/prd/epic-011-mini-payment-refund-flow.md

@@ -263,6 +263,49 @@ class PaymentMtService {
 - **2支付成功** → **3已退款** (退款成功)
 - **1支付中** → **4支付失败** (支付失败)
 
+### 支付退款流程时序图
+
+```mermaid
+sequenceDiagram
+    participant User as 用户
+    participant Mini as Mini小程序
+    participant OrderAPI as 订单API
+    participant PaymentAPI as 支付API
+    participant WechatPay as 微信支付
+    participant RefundAPI as 退款API
+
+    %% 支付流程
+    User->>Mini: 提交订单
+    Mini->>OrderAPI: 创建订单
+    OrderAPI-->>Mini: 返回订单ID
+    Mini->>PaymentAPI: 请求支付参数
+    PaymentAPI-->>Mini: 返回支付参数
+    Mini->>WechatPay: 调用微信支付
+    WechatPay-->>Mini: 支付结果
+
+    %% 支付回调
+    WechatPay->>PaymentAPI: 支付回调通知
+    PaymentAPI->>OrderAPI: 更新订单状态(已支付)
+    OrderAPI-->>PaymentAPI: 状态更新成功
+    PaymentAPI-->>WechatPay: 回调响应
+
+    %% 退款流程
+    User->>Mini: 取消订单
+    Mini->>OrderAPI: 请求取消订单
+    OrderAPI->>RefundAPI: 发起退款
+    RefundAPI->>WechatPay: 调用退款API
+    WechatPay-->>RefundAPI: 退款结果
+    RefundAPI->>OrderAPI: 更新订单状态(已退款)
+    OrderAPI-->>Mini: 取消成功
+    Mini-->>User: 显示取消结果
+
+    %% 退款回调
+    WechatPay->>RefundAPI: 退款回调通知
+    RefundAPI->>OrderAPI: 确认退款状态
+    OrderAPI-->>RefundAPI: 状态确认成功
+    RefundAPI-->>WechatPay: 回调响应
+```
+
 ### Mini小程序前端实现
 ```typescript
 // 支付页面 - 需要创建 /pages/payment/index.tsx