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

[italomatos]

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.

fizzy(dev):002> Card.last.events.order(:created_at).last
  CACHE Card Load (0.1ms)  SELECT "cards".* FROM "cards" ORDER BY "cards"."id" DESC LIMIT ?  [["LIMIT", 1]]
  Event Load (1.0ms)  SELECT "events".* FROM "events" WHERE "events"."eventable_id" = ? AND "events"."eventable_type" = ? ORDER BY "events"."created_at" DESC LIMIT ?  [["eventable_id", "<16 bytes of binary data>"], ["eventable_type", "Card"], ["LIMIT", 1]]

fizzy(dev):001> Card.last.events.order(created_at: :desc).first
  Card Load (0.3ms)  SELECT "cards".* FROM "cards" ORDER BY "cards"."id" DESC LIMIT ?  [["LIMIT", 1]]
  Event Load (0.8ms)  SELECT "events".* FROM "events" WHERE "events"."eventable_id" = ? AND "events"."eventable_type" = ? ORDER BY "events"."created_at" DESC LIMIT ?  [["eventable_id", "<16 bytes of binary data>"], ["eventable_type", "Card"], ["LIMIT", 1]]