feat: 管理首页新增会话存储后端状态展示

- 仪表盘新增「会话存储后端」卡片，显示 Redis/内存 模式
- 降级时显示「已降级」标签 + 降级说明
- 内存模式下显示活跃会话数
- 新增 stat-red / stat-desc / stat-badge / badge-warn CSS 样式

internal/controller/admin/admin_controller.go

@@ -29,9 +29,15 @@ func (ac *AdminController) Dashboard(c *gin.Context) {
 	if err != nil {
 		totalUsers = 0
 	}
+	storeMetrics := ac.adminService.GetStoreMetrics()
 	c.HTML(http.StatusOK, "admin/dashboard/index.html", common.BuildAdminPageData(c, gin.H{
-		"Title":      "管理首页",
-		"TotalUsers": totalUsers,
+		"Title":          "管理首页",
+		"TotalUsers":     totalUsers,
+		"StoreType":      storeMetrics.Type,
+		"StoreHealthy":   storeMetrics.Healthy,
+		"StoreDegraded":  storeMetrics.Degraded,
+		"DegradedNote":   storeMetrics.DegradedNote,
+		"ActiveSessions": storeMetrics.ActiveCount,
 	}))
 }