Better representation of inoperative dining schedules

[laggycomputer]

Currently, when a period doesn't operate on a date in a dining hall, the aggregate on the restaurantToday endpoint looks like this:

  "4180cffd-2823-4a39-839e-612c74187b11": {
    "name": "Afternoon Snack",
    "startTime": "14:30:00",
    "endTime": "16:30:00",
    "stationToDishes": {
      "1872": [],
      "1875": [],
      "1878": [],
      "1881": [],
      "1884": [],
      "1887": [],
      "1890": [],
      "1893": [],
      "1896": [],
      "1899": [],
      "2166": [],
      "275501": []
    },
    "updatedAt": "2026-06-02T19:01:13.293Z"
  },

which looks ugly, and is ambiguous.
Let's find a better way...