Refactor: use idiomatic .last instead of .order(:desc).first (#2098)

Simplifies the last_event method in ActivitySpike::Detector by using
the more idiomatic Rails pattern .order(:created_at).last instead of
.order(created_at: :desc).first. Both generate the same SQL query but
.last is more readable and conventional in Rails codebases.

app/models/card/activity_spike/detector.rb

@@ -51,6 +51,6 @@ class Card::ActivitySpike::Detector
     end
 
     def last_event
-      card.events.order(created_at: :desc).first
+      card.events.order(:created_at).last
     end
 end