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/export.go

@@ -10,6 +10,7 @@ import (
 	"github.com/gin-gonic/gin"
 	"kefu-cloud/server/internal/middleware"
 	"kefu-cloud/server/internal/model"
+	"kefu-cloud/server/internal/service"
 )
 
 // Export 导出客户列表 CSV（沿用列表筛选与坐席可见范围）。
@@ -128,7 +129,7 @@ func (h *SessionHandler) Export(c *gin.Context) {
 	customerMap := map[uint]model.Customer{}
 	if len(customerIDs) > 0 {
 		var customers []model.Customer
-		model.DB.Where("id IN ?", uniqueUint(customerIDs)).Find(&customers)
+		model.DB.Where("id IN ?", service.UniqueUint(customerIDs)).Find(&customers)
 		for _, cu := range customers {
 			customerMap[cu.ID] = cu
 		}
@@ -136,7 +137,7 @@ func (h *SessionHandler) Export(c *gin.Context) {
 	agentMap := map[uint]model.User{}
 	if len(agentIDs) > 0 {
 		var users []model.User
-		model.DB.Where("id IN ?", uniqueUint(agentIDs)).Find(&users)
+		model.DB.Where("id IN ?", service.UniqueUint(agentIDs)).Find(&users)
 		for _, u := range users {
 			agentMap[u.ID] = u
 		}
@@ -144,7 +145,7 @@ func (h *SessionHandler) Export(c *gin.Context) {
 	channelMap := map[uint]model.Channel{}
 	if len(channelIDs) > 0 {
 		var channels []model.Channel
-		model.DB.Where("id IN ?", uniqueUint(channelIDs)).Find(&channels)
+		model.DB.Where("id IN ?", service.UniqueUint(channelIDs)).Find(&channels)
 		for _, ch := range channels {
 			channelMap[ch.ID] = ch
 		}