Add created_at/updated_at to PetState

This has just been absent for too long! We've lost a lot of data about
when poses were first modeled, which is a shame.

But I want this in now, because I was just doing caching on
/rainbow-pool.json, and realized that _labeling_ poses is another way
pet states can update rather than just being created!

So we need an `updated_at` field, to be able to quickly detect edits
that require us to invalidate the cache on
`PetState.all_supported_poses`. I'll do that next!

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