API: Expose `Hackathon.pending.count` for Bank admin dashboard

[Gary Tou]

The Hack Club Bank admin dashboard currently contains a card/link to the Airtable with the pending count (from Airtable).

We should expose the pending count via the API so that the card can reflect the right count.

---

/api/v1/stats/hackathons

{
  "status": {
    "pending": {
      "meta": {
        "count": 1
      }
    }
  }
}