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.

Start by examining how the `stuff` database currently represents item fields and compare it with the `game-night` entry's `Players` field. Add a quantity for available units such as monitors and arduinos, then verify that the value is stored and shown wherever item details are presented.

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