feat(payment): 添加付款用途常量定义并更新数据结构

- 定义 PaymentPurpose 类型和相关常量值
- 在 ERP 服务中添加 Payment 模块引用
- 更新 Payment 结构体中的 PaymentPurpose 字段类型
- 将字符串类型替换为常量类型以增强类型安全
- 添加采购付款单、费用报销和其他支出的用途常量

ik3cloud/constant/constant.go

@@ -79,6 +79,14 @@ const (
 	PaymentTypeRequest  PaymentType = "FKDLX11_SYS" // 报销付款单
 )
 
+type PaymentPurpose = string
+
+const (
+	PaymentPurposePurchase PaymentPurpose = "SFKYT08_SYS" // 采购付款单
+	PaymentPurposeExpense  PaymentPurpose = "SFKYT009"    // 费用报销
+	PaymentPurposeOther    PaymentPurpose = "SFKYT12_SYS" // 其他支出
+)
+
 type ReceiptType = string
 
 const (