feat(admin): 分组管理新增容量列（并发/会话/RPM 实时聚合）

复用 GroupCapacityService，在 admin 分组列表中添加容量列，
显示每个分组的实时并发/会话/RPM 使用量和上限。

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

backend/internal/handler/sora_gateway_handler_test.go

@@ -348,6 +348,9 @@ func (s *stubUsageLogRepo) GetGroupStatsWithFilters(ctx context.Context, startTi
 func (s *stubUsageLogRepo) GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) ([]usagestats.UserBreakdownItem, error) {
 	return nil, nil
 }
+func (s *stubUsageLogRepo) GetAllGroupUsageSummary(ctx context.Context, todayStart time.Time) ([]usagestats.GroupUsageSummary, error) {
+	return nil, nil
+}
 func (s *stubUsageLogRepo) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
 	return nil, nil
 }