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/customer.go

@@ -7,6 +7,7 @@ import (
 	"github.com/gin-gonic/gin"
 	"kefu-cloud/server/internal/middleware"
 	"kefu-cloud/server/internal/model"
+	"kefu-cloud/server/internal/service"
 )
 
 type CustomerHandler struct{}
@@ -85,7 +86,7 @@ func (h *CustomerHandler) Get(c *gin.Context) {
 		sessionQuery = sessionQuery.Where("agent_id = ?", middleware.GetUserID(c))
 	}
 	sessionQuery.Order("created_at desc").Limit(20).Find(&sessions)
-	contacts := listCustomerContacts(customer.ID)
+	contacts := service.ListCustomerContacts(customer.ID)
 
 	middleware.JSON(c, gin.H{
 		"customer": customer,
@@ -207,18 +208,18 @@ func (h *CustomerHandler) Create(c *gin.Context) {
 	}
 	syncCustomerPhones(tenantID, customer.ID, phones)
 	// 主字段微信/邮箱/QQ 也写入 contacts（便于统一展示）
-	var extras []extractedContact
+	var extras []service.ExtractedContact
 	if customer.Wechat != "" {
-		extras = append(extras, extractedContact{Kind: "wechat", Value: customer.Wechat})
+		extras = append(extras, service.ExtractedContact{Kind: "wechat", Value: customer.Wechat})
 	}
 	if customer.QQ != "" {
-		extras = append(extras, extractedContact{Kind: "qq", Value: customer.QQ})
+		extras = append(extras, service.ExtractedContact{Kind: "qq", Value: customer.QQ})
 	}
 	if customer.Email != "" {
-		extras = append(extras, extractedContact{Kind: "email", Value: customer.Email})
+		extras = append(extras, service.ExtractedContact{Kind: "email", Value: customer.Email})
 	}
 	if len(extras) > 0 {
-		_, _ = mergeCustomerContacts(tenantID, customer.ID, extras, "agent")
+		_, _ = service.MergeCustomerContacts(tenantID, customer.ID, extras, "agent")
 	}
 
 	middleware.JSON(c, customer)