Set member status to sustainer upon payment

Introduces a state machine for the payment status as well.

refs #213

spec/requests/contributions/donations_spec.rb

@@ -177,7 +177,7 @@ RSpec.describe "Donations", type: :request do
             .to_return(status: 200, headers: {}, body: invoice)
           stub_request(:get, "http://btcpay.example.com/api/v1/stores/123456/invoices/MCkDbf2cUgBuuisUCgnRnb/payment-methods")
             .to_return(status: 200, headers: {}, body: payments)
-
+          allow(user).to receive(:add_member_status).with(:sustainer).and_return(["sustainer"])
           get confirm_btcpay_contributions_donation_path(subject)
         end
 
@@ -185,11 +185,16 @@ RSpec.describe "Donations", type: :request do
           subject.reload
           expect(subject.paid_at).not_to be_nil
           expect(subject.amount_sats).to eq(2061)
+          expect(subject.payment_status).to eq("settled")
         end
 
         it "redirects to the donations index" do
           expect(response).to redirect_to(contributions_donations_url)
         end
+
+        it "updates the user's member status" do
+          expect(user).to have_received(:add_member_status).with(:sustainer)
+        end
       end
 
       describe "amount in sats" do