Use PetState#updated_at for the supported pose cache key, not latest ID

This is because labeling poses with the Support tools *should*
invalidate the `PetState.all_supported_poses` cache! But the previous
cache key would only invalidate when a new pet state is *added*, not
when one is *edited*.

app/models/pet_state.rb

@@ -211,7 +211,7 @@ class PetState < ApplicationRecord
   end
 
   def self.last_updated_key
-    PetState.maximum(:id)
+    PetState.maximum(:updated_at)
   end
 
   def self.all_supported_poses

db/migrate/20241008004715_add_timestamps_to_pet_states.rb

@@ -0,0 +1,5 @@
+class AddTimestampsToPetStates < ActiveRecord::Migration[7.2]
+  def change
+    add_timestamps :pet_states, null: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2024_10_03_004726) do
+ActiveRecord::Schema[7.2].define(version: 2024_10_08_004715) do
   create_table "alt_styles", charset: "utf8mb4", collation: "utf8mb4_unicode_520_ci", force: :cascade do |t|
     t.integer "species_id", null: false
     t.integer "color_id", null: false
@@ -219,6 +219,8 @@ ActiveRecord::Schema[7.2].define(version: 2024_10_03_004726) do
     t.boolean "labeled", default: false, null: false
     t.boolean "glitched", default: false, null: false
     t.string "artist_neopets_username"
+    t.datetime "created_at"
+    t.datetime "updated_at"
     t.index ["pet_type_id"], name: "pet_states_pet_type_id"
   end