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,
+		"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）

internal/service/admin_service.go

@@ -112,6 +112,11 @@ func (s *AdminService) CountUsers() (int64, error) {
 	return s.userRepo.CountSearchUsers("", "", "")
 }
 
+// GetStoreMetrics 返回会话存储状态信息（管理首页用）
+func (s *AdminService) GetStoreMetrics() session.StoreMetrics {
+	return s.sessionManager.GetStoreMetrics()
+}
+
 // checkAndOperate 通用权限检查 + 执行操作
 func (s *AdminService) checkAndOperate(operatorUID, targetUID uint, operate func(*model.User, *model.User) error) error {
 	// 1. 不可操作自己

templates/admin/html/dashboard/index.html

@@ -19,6 +19,23 @@
             </div>
         </div>
 
+        <div class="stats-grid" style="margin-top: 1rem;">
+            <div class="stat-card" style="grid-column: span 2;">
+                <div class="stat-icon {{if .StoreDegraded}}stat-red{{else if eq .StoreType "redis"}}stat-green{{else}}stat-blue{{end}}">
+                    <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><ellipse cx="12" cy="5" rx="9" ry="3"/><path d="M21 12c0 1.66-4 3-9 3s-9-1.34-9-3"/><path d="M3 5v14c0 1.66 4 3 9 3s9-1.34 9-3V5"/></svg>
+                </div>
+                <div class="stat-info">
+                    <div class="stat-label">会话存储后端</div>
+                    <div class="stat-value">
+                        {{if eq .StoreType "redis"}}Redis{{else}}内存{{end}}
+                        {{if .StoreDegraded}}<span class="stat-badge badge-warn">已降级</span>{{end}}
+                    </div>
+                    {{if .DegradedNote}}<div class="stat-desc">{{.DegradedNote}}</div>{{end}}
+                    {{if ge .ActiveSessions 0}}<div class="stat-desc">活跃会话数：{{.ActiveSessions}}</div>{{end}}
+                </div>
+            </div>
+        </div>
+
         <div class="info-card">
             <h3>快速开始</h3>
             <ul>

templates/admin/static/css/common.css

@@ -138,8 +138,12 @@ a:hover { color: var(--primary-hover); }
 .stat-blue { background: #e3f2fd; color: var(--primary); }
 .stat-green { background: #e8f5e9; color: var(--success); }
 .stat-orange { background: #fff3e0; color: var(--warning); }
+.stat-red { background: #ffebee; color: var(--danger, #e53935); }
 .stat-label { font-size: 12px; color: var(--text-muted); }
 .stat-value { font-size: 22px; font-weight: 600; color: var(--text); }
+.stat-desc { font-size: 12px; color: var(--text-muted); margin-top: 4px; }
+.stat-badge { display: inline-block; font-size: 11px; padding: 1px 8px; border-radius: 10px; margin-left: 8px; vertical-align: middle; }
+.badge-warn { background: #fff3e0; color: var(--warning); }
 
 /* --- 信息卡片 --- */
 .info-card {