feat(admin): 分组管理列表新增用量列与账号数分类

分组管理列表增强：

1. 今日/累计用量列：
   - 新增独立端点 GET /admin/groups/usage-summary
   - 一次查询返回所有分组的今日费用和累计费用（actual_cost）
   - 前端异步加载后合并显示在分组列表中

2. 账号数区分可用/限流/总量：
   - 将账号数列从单一总量改为 badge 内多行展示
   - 可用: active + schedulable 的账号数（绿色）
   - 限流: rate_limit/overload/temp_unschedulable 的账号数（橙色，无限流时隐藏）
   - 总量: 全部关联账号数

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/handler/admin/group_handler.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
 	"github.com/gin-gonic/gin"
@@ -16,7 +17,8 @@ import (
 
 // GroupHandler handles admin group management
 type GroupHandler struct {
-	adminService service.AdminService
+	adminService     service.AdminService
+	dashboardService *service.DashboardService
 }
 
 type optionalLimitField struct {
@@ -69,9 +71,10 @@ func (f optionalLimitField) ToServiceInput() *float64 {
 }
 
 // NewGroupHandler creates a new admin group handler
-func NewGroupHandler(adminService service.AdminService) *GroupHandler {
+func NewGroupHandler(adminService service.AdminService, dashboardService *service.DashboardService) *GroupHandler {
 	return &GroupHandler{
-		adminService: adminService,
+		adminService:     adminService,
+		dashboardService: dashboardService,
 	}
 }
 
@@ -363,6 +366,22 @@ func (h *GroupHandler) GetStats(c *gin.Context) {
 	_ = groupID // TODO: implement actual stats
 }
 
+// GetUsageSummary returns today's and cumulative cost for all groups.
+// GET /api/v1/admin/groups/usage-summary?timezone=Asia/Shanghai
+func (h *GroupHandler) GetUsageSummary(c *gin.Context) {
+	userTZ := c.Query("timezone")
+	now := timezone.NowInUserLocation(userTZ)
+	todayStart := timezone.StartOfDayInUserLocation(now, userTZ)
+
+	results, err := h.dashboardService.GetGroupUsageSummary(c.Request.Context(), todayStart)
+	if err != nil {
+		response.Error(c, 500, "Failed to get group usage summary")
+		return
+	}
+
+	response.Success(c, results)
+}
+
 // GetGroupAPIKeys handles getting API keys in a group
 // GET /api/v1/admin/groups/:id/api-keys
 func (h *GroupHandler) GetGroupAPIKeys(c *gin.Context) {