支持客户、对话记录与统计报告 CSV 导出

- 新增 /customers/export、/sessions/export、/statistics/export
- 沿用列表筛选与角色可见范围，UTF-8 BOM 便于 Excel 打开
- 前端三处导出按钮接入真实下载

server/internal/handler/router.go

@@ -53,6 +53,7 @@ func SetupRoutes(r *gin.Engine, store storage.ObjectStorage, storageCfg config.S
 		authRequired.GET("/agents/available", session.ListAvailableAgents)
 		sessions := authRequired.Group("/sessions")
 		sessions.GET("", session.List)
+		sessions.GET("/export", session.Export)
 		sessions.GET("/:id", session.Get)
 		sessions.POST("", session.Create)
 		sessions.POST("/batch-archive", session.BatchArchive)
@@ -69,6 +70,7 @@ func SetupRoutes(r *gin.Engine, store storage.ObjectStorage, storageCfg config.S
 		// 客户管理
 		customers := authRequired.Group("/customers")
 		customers.GET("", customer.List)
+		customers.GET("/export", customer.Export)
 		customers.GET("/:id", customer.Get)
 		customers.POST("", customer.Create)
 		customers.PUT("/:id", customer.Update)
@@ -110,6 +112,7 @@ func SetupRoutes(r *gin.Engine, store storage.ObjectStorage, storageCfg config.S
 		statistics.GET("/response-distribution", stats.ResponseDistribution)
 		statistics.GET("/performance", stats.AgentPerformance)
 		statistics.GET("/channels", stats.ChannelDistribution)
+		statistics.GET("/export", stats.Export)
 
 		// 管理端接口（需要管理员权限）
 		adminGroup := authRequired.Group("/admin")