Accept diff-only for PATCH expo /category-groups/id #241

refer to HackGT/timber#266 for the UI side changes

currently we have

const updatedCategoryGroup = await prisma.categoryGroup.update({
  //...
  data: {
    //...
    users: {
      set: req.body.users?.map((id: number) => ({ id })) ?? undefined,
    },
  },
});

(code ptr: services/expo/src/routes/categorygroups.ts, in categoryGroupRoutes.route("/:id").patch)

which takes the entire body.users (list of users) and sets it into storage which could cause a race condition if multiple people are adding judges.

Change this to accept the diff from the UI and instead perform a merge on the current set of users.
Talk to whoevers working on the ui side (or yourself) to confirm data shape, probably do something like {add: [...], removed: [...]} and then we just have to fetch the current list of judges and merge it in

^ There maybe could be a race condition between endpoint handles too (another call pulls users list before a previous one is done writing back in to db), so might have to deal with that too (lower prio). Maybe theres a postgres thing for that.