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,
 	}))
 }
 

internal/controller/admin/interfaces.go

@@ -3,14 +3,16 @@ package admin
 import (
 	"metazone.cc/metalab/internal/model"
 	"metazone.cc/metalab/internal/service"
+	"metazone.cc/metalab/internal/session"
 )
 
-// adminUseCase AdminController 对 AdminService 的最小依赖（ISP：4 个方法）
+// adminUseCase AdminController 对 AdminService 的最小依赖（ISP：5 个方法）
 type adminUseCase interface {
 	ListUsers(params service.ListUsersParams) (*service.ListUsersResult, error)
 	UpdateUserStatus(operatorUID, targetUID uint, newStatus string) error
 	ResetUserToken(operatorUID, targetUID uint) error
 	CountUsers() (int64, error)
+	GetStoreMetrics() session.StoreMetrics
 }
 
 // auditUseCase AuditController 对 AuditService 的最小依赖（ISP：3 个方法）