🥗🧹 Tobias: Define Payment relationships and attributes

This is not super important to the actual feature, but it does mean that
the `Payment` has clearly defined relationships, with tests that confirm
the relationship.

It also adds a spec to check that the `#amount` value operates as a
monetized value; which doesn't seem like it does that much beyond giving
us an injection point for layering in additional details.

app/furniture/tobias/payment.rb

@@ -2,7 +2,7 @@ class Tobias
   class Payment < ApplicationRecord
     self.table_name = "tobias_payments"
 
-    belongs_to :payout
+    belongs_to :payout, inverse_of: :payments
 
     monetize :amount_cents
   end

spec/spec_helper.rb

@@ -1,6 +1,7 @@
 require "faker"
 require "pundit/rspec"
 require "simplecov"
+require "money-rails/test_helpers"
 
 SimpleCov.start do
   enable_coverage :branch

spec/tobias/payment_spec.rb

@@ -0,0 +1,11 @@
+require "rails_helper"
+
+RSpec.describe Tobias::Payment, type: :model do
+  describe "#payout" do
+    it { is_expected.to belong_to(:payout).inverse_of(:payments) }
+  end
+
+  describe "#amount" do
+    it { is_expected.to monetize(:amount) }
+  end
+end