Use dynamic list for allowed user preference params

spec/models/user_preferences_spec.rb

@@ -38,4 +38,15 @@ RSpec.describe UserPreferences, type: :model do
       expect(res['lightning_notify_sats_received_threshold']).to eq(1000)
     end
   end
+
+  describe ".pref_keys" do
+    let(:default_prefs) { YAML.load_file("#{Rails.root}/config/default_preferences.yml") }
+
+    it "returns the keys of all default preferences as an array of symbols" do
+      expect(UserPreferences.pref_keys).to be_a(Array)
+      expect(UserPreferences.pref_keys).to include(:lightning_notify_sats_received)
+      expect(UserPreferences.pref_keys).to include(:xmpp_exchange_contacts_with_invitees)
+      expect(UserPreferences.pref_keys.length).to eq(default_prefs.keys.length)
+    end
+  end
 end