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/router.go

@@ -6,6 +6,7 @@ import (
 	"github.com/gin-gonic/gin"
 	"kefu-cloud/server/internal/config"
 	"kefu-cloud/server/internal/middleware"
+	"kefu-cloud/server/internal/service"
 	"kefu-cloud/server/internal/storage"
 	"kefu-cloud/server/internal/ws"
 )
@@ -28,9 +29,9 @@ func SetupRoutes(r *gin.Engine, store storage.ObjectStorage, storageCfg config.S
 	upload := NewUploadHandler(store, storageCfg)
 	perm := NewPermissionHandler()
 
-	SetImagePublicBase(storageCfg.PublicBase)
+	service.SetImagePublicBase(storageCfg.PublicBase)
 	// 访客输入草稿 → 提取联系方式（由 ws 包回调，避免循环依赖）
-	ws.ProcessDraftContacts = onVisitorDraftContacts
+	ws.ProcessDraftContacts = service.OnVisitorDraftContacts
 
 	api := r.Group("/api")