Fix instance actor being incorrectly created when running migrations (#18109)

* Add migration test about instance actor key

* Fix old migration

* Work around incorrect database state

app/models/account.rb

@@ -559,6 +559,12 @@ class Account < ApplicationRecord
   before_validation :prepare_username, on: :create
   before_destroy :clean_feed_manager
 
+  def ensure_keys!
+    return unless local? && private_key.blank? && public_key.blank?
+    generate_keys
+    save!
+  end
+
   private
 
   def prepare_contents

app/models/concerns/account_finder_concern.rb

@@ -13,7 +13,7 @@ module AccountFinderConcern
     end
 
     def representative
-      Account.find(-99)
+      Account.find(-99).tap(&:ensure_keys!)
     rescue ActiveRecord::RecordNotFound
       Account.create!(id: -99, actor_type: 'Application', locked: true, username: Rails.configuration.x.local_domain)
     end