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.

Written by the indexing model from the issue text.