add tests for controllers and ActionCable channels; improve main backend docs

main-server/spec/jobs/matchmaking_sweep_job_spec.rb

@@ -0,0 +1,10 @@
+require "rails_helper"
+
+RSpec.describe MatchmakingSweepJob, type: :job do
+  describe "#perform" do
+    it "calls MatchmakingService.sweep" do
+      expect(MatchmakingService).to receive(:sweep)
+      described_class.new.perform
+    end
+  end
+end