feat(admin/usage): 优化管理员用量页面功能和体验

后端改进：
- 新增 GetStatsWithFilters 方法支持完整筛选条件
- Stats 端点支持 account_id, group_id, model, stream, billing_type 参数
- 统一使用 filters 结构体，移除冗余的分支逻辑

前端改进：
- 统计卡片添加"所选范围内"文字提示
- 优化总消费显示格式，清晰展示实际费用和标准计费
- Token 和费用列添加问号图标 tooltip 显示详细信息
- API Key 搜索框体验优化：点击即显示下拉选项
- 选择用户后自动加载该用户的所有 API Key

backend/internal/repository/usage_log_repo.go

@@ -1388,6 +1388,81 @@ func (r *usageLogRepository) GetGlobalStats(ctx context.Context, startTime, endT
 	return stats, nil
 }
 
+// GetStatsWithFilters gets usage statistics with optional filters
+func (r *usageLogRepository) GetStatsWithFilters(ctx context.Context, filters UsageLogFilters) (*UsageStats, error) {
+	conditions := make([]string, 0, 9)
+	args := make([]any, 0, 9)
+
+	if filters.UserID > 0 {
+		conditions = append(conditions, fmt.Sprintf("user_id = $%d", len(args)+1))
+		args = append(args, filters.UserID)
+	}
+	if filters.APIKeyID > 0 {
+		conditions = append(conditions, fmt.Sprintf("api_key_id = $%d", len(args)+1))
+		args = append(args, filters.APIKeyID)
+	}
+	if filters.AccountID > 0 {
+		conditions = append(conditions, fmt.Sprintf("account_id = $%d", len(args)+1))
+		args = append(args, filters.AccountID)
+	}
+	if filters.GroupID > 0 {
+		conditions = append(conditions, fmt.Sprintf("group_id = $%d", len(args)+1))
+		args = append(args, filters.GroupID)
+	}
+	if filters.Model != "" {
+		conditions = append(conditions, fmt.Sprintf("model = $%d", len(args)+1))
+		args = append(args, filters.Model)
+	}
+	if filters.Stream != nil {
+		conditions = append(conditions, fmt.Sprintf("stream = $%d", len(args)+1))
+		args = append(args, *filters.Stream)
+	}
+	if filters.BillingType != nil {
+		conditions = append(conditions, fmt.Sprintf("billing_type = $%d", len(args)+1))
+		args = append(args, int16(*filters.BillingType))
+	}
+	if filters.StartTime != nil {
+		conditions = append(conditions, fmt.Sprintf("created_at >= $%d", len(args)+1))
+		args = append(args, *filters.StartTime)
+	}
+	if filters.EndTime != nil {
+		conditions = append(conditions, fmt.Sprintf("created_at <= $%d", len(args)+1))
+		args = append(args, *filters.EndTime)
+	}
+
+	query := fmt.Sprintf(`
+		SELECT
+			COUNT(*) as total_requests,
+			COALESCE(SUM(input_tokens), 0) as total_input_tokens,
+			COALESCE(SUM(output_tokens), 0) as total_output_tokens,
+			COALESCE(SUM(cache_creation_tokens + cache_read_tokens), 0) as total_cache_tokens,
+			COALESCE(SUM(total_cost), 0) as total_cost,
+			COALESCE(SUM(actual_cost), 0) as total_actual_cost,
+			COALESCE(AVG(duration_ms), 0) as avg_duration_ms
+		FROM usage_logs
+		%s
+	`, buildWhere(conditions))
+
+	stats := &UsageStats{}
+	if err := scanSingleRow(
+		ctx,
+		r.sql,
+		query,
+		args,
+		&stats.TotalRequests,
+		&stats.TotalInputTokens,
+		&stats.TotalOutputTokens,
+		&stats.TotalCacheTokens,
+		&stats.TotalCost,
+		&stats.TotalActualCost,
+		&stats.AverageDurationMs,
+	); err != nil {
+		return nil, err
+	}
+	stats.TotalTokens = stats.TotalInputTokens + stats.TotalOutputTokens + stats.TotalCacheTokens
+	return stats, nil
+}
+
 // AccountUsageHistory represents daily usage history for an account
 type AccountUsageHistory = usagestats.AccountUsageHistory