feat(admin): Agent 并发 - 后台可调同时对话上限并展示 Agent 服务状态

新增运行策略 agentMaxSessions（默认 30，范围 1-64），保存后同时下发到内嵌进程闸门与独立 yingce-agent 的 /v1/limit，降低上限不会中断已开始的对话。系统性能页展示 Agent 服务健康、进行中/排队数量，并可直接修改上限。

backend/internal/handler/admin_system_performance.go

@@ -23,6 +23,26 @@ func RegisterAdminSystemPerformanceRoutes(r *gin.RouterGroup, svc *service.Servi
 		ok(c, result)
 	})
 
+	r.PUT("/admin/system-performance/agent-limit", func(c *gin.Context) {
+		actor, err := currentUser(c, svc)
+		if err != nil {
+			failService(c, err)
+			return
+		}
+		c.Request.Body = http.MaxBytesReader(c.Writer, c.Request.Body, 4<<10)
+		var request service.AgentSessionLimitRequest
+		if err := c.ShouldBindJSON(&request); err != nil {
+			failService(c, service.BadAuthRequest("Agent 并发请求无效"))
+			return
+		}
+		result, err := svc.UpdateAgentSessionLimit(c.Request.Context(), actor, request.MaxSessions)
+		if err != nil {
+			failService(c, err)
+			return
+		}
+		ok(c, result)
+	})
+
 	r.POST("/admin/system-performance/cache/clear", func(c *gin.Context) {
 		actor, err := currentUser(c, svc)
 		if err != nil {

backend/internal/handler/admin_system_performance_test.go

@@ -14,6 +14,7 @@ func TestAdminSystemPerformanceRoutesAreRegistered(t *testing.T) {
 	RegisterAdminSystemPerformanceRoutes(router.Group("/api"), &service.Service{})
 	wanted := map[string]bool{
 		"GET /api/admin/system-performance":              false,
+		"PUT /api/admin/system-performance/agent-limit":  false,
 		"POST /api/admin/system-performance/cache/clear": false,
 	}
 	for _, route := range router.Routes() {