Add numeric_ap_ids feature flag to switch new accounts to numeric AP identifiers (#36316)

app/models/account.rb

@@ -446,6 +446,7 @@ class Account < ApplicationRecord
 
   before_validation :prepare_contents, if: :local?
   before_create :generate_keys
+  before_create :set_id_scheme
   before_destroy :clean_feed_manager
 
   def ensure_keys!
@@ -470,6 +471,12 @@ class Account < ApplicationRecord
     self.public_key  = keypair.public_key.to_pem
   end
 
+  def set_id_scheme
+    return unless local? && Mastodon::Feature.numeric_ap_ids_enabled?
+
+    self.id_scheme = :numeric_ap_id
+  end
+
   def normalize_domain
     return if local?