FIX: prevents readonly mode to crash channel_messages#index (#22987)

Prior to this fix `context.membership&.update!(last_viewed_at: Time.zone.now)` would generate an update statement from a GET request which is not permitted by default when in readonly mode.

The usual fix in this case is to check for readonly or rescue an error, however, this common pattern of updating "last seen" or similar can be better handled in a `Schedule::Defer` block, which won't raise the `ActiveRecord::ReadOnlyError` when in readonly and will also prevent the controller to wait for this operation.

plugins/chat/spec/requests/chat/api/channel_messages_controller_spec.rb

@@ -28,6 +28,19 @@ RSpec.describe Chat::Api::ChannelMessagesController do
       end
     end
 
+    context "when readonly mode" do
+      fab!(:message_1) { Fabricate(:chat_message, chat_channel: channel) }
+
+      before { Discourse.enable_readonly_mode }
+      after { Discourse.disable_readonly_mode }
+
+      it "works" do
+        get "/chat/api/channels/#{channel.id}/messages"
+
+        expect(response.status).to eq(200)
+      end
+    end
+
     context "when channnel doesn’t exist" do
       it "returns a 404" do
         get "/chat/api/channels/-999/messages"