refactor(server): 会话/访客/设置业务逻辑下沉 internal/service 层，handler 瘦身

- handler 只保留请求绑定、权限检查与响应映射；业务校验顺序、状态码与文案逐字保留
- 新增 service.Error 语义错误，由 handler 统一映射 HTTP 响应
- session 875→361 行、widget 745→321 行、settings 540→73 行
- 自动分配/租户设置(工作时间、欢迎语)/消息校验/黑名单检查/联系方式提取随迁 service 包
- 相关单元测试随迁 service 包，go test ./... 全部通过

server/internal/handler/upload.go

@@ -12,12 +12,13 @@ import (
 	"kefu-cloud/server/internal/config"
 	"kefu-cloud/server/internal/media"
 	"kefu-cloud/server/internal/middleware"
+	"kefu-cloud/server/internal/service"
 	"kefu-cloud/server/internal/storage"
 )
 
 type UploadHandler struct {
-	store  storage.ObjectStorage
-	cfg    config.StorageConfig
+	store storage.ObjectStorage
+	cfg   config.StorageConfig
 }
 
 func NewUploadHandler(store storage.ObjectStorage, cfg config.StorageConfig) *UploadHandler {
@@ -45,8 +46,9 @@ func (h *UploadHandler) WidgetUploadImage(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "会话参数错误"})
 		return
 	}
-	session, ok := loadVisitorSession(c, sid, token)
-	if !ok {
+	session, err := service.WidgetSession(sid, token)
+	if err != nil {
+		respondServiceError(c, err)
 		return
 	}
 	if session.Status == "ended" || session.Status == "archived" {