Add a `quantity` field

Would be good if we could have a rough quantity of available units for common stuff like monitors and arduinos.
Shouldn't be too hard. `game-night` had `Players.` Basically just re-add it.

首先检查 `stuff` 数据库目前如何表示项目字段，并将其与 `game-night` 条目的 `Players` 字段进行比较。为显示器和 arduinos 等可用单元添加数量，然后验证该值是否在显示项目详细信息的所有位置都得到存储和显示。

由索引模型根据 Issue 内容生成。