feat: 新增配送合同附件安全上传功能

- 增加合同专用 PDF 上传、鉴权预览和失败清理接口
- 支持草稿附件替换移除、并发保护和启用完整性校验
- 修复循环模板引用导致文件选择器无法打开的问题
- 补充专项测试、中文项目文档和操作日志

backend/api/internal/routers/platform.go

@@ -70,8 +70,11 @@ func registerGasorderRoute(group *gin.RouterGroup) {
 	contract := group.Group("/gasorder_contract")
 	contract.GET("", gasorder.ListGasorderContract)
 	contract.POST("", gasorder.CreateGasorderContract)
+	contract.POST("/attachment/upload", gasorder.UploadGasorderContractAttachment)
+	contract.POST("/attachment/cleanup", gasorder.CleanupGasorderContractAttachment)
 	contract.GET("/:identity", gasorder.GetGasorderContract)
 	contract.PUT("/:identity", gasorder.UpdateGasorderContract)
+	contract.GET("/:identity/attachment", gasorder.ServeGasorderContractAttachment)
 	contract.POST("/:identity/activate", gasorder.ActivateGasorderContract)
 	contract.POST("/:identity/renew", gasorder.RenewGasorderContract)
 	contract.POST("/:identity/terminate", gasorder.TerminateGasorderContract)