Copilot payments: Fix the schema

[jmgasper]

From the recent review:

 const schema = new mongoose.Schema({
  project: { type: String, required: true },
  amount: { type: Number, required: true },
  description: { type: String, required: true },
  challenge: { type: Number, required: true },
  closed: { type: String, required: true, default: "false" },
}, {

You also need to persist challenge id as part of the model as per spec. challenge should be renamed challengeId