Auto-increment external_account_id automatically by default to allow upcoming simple signups

db/migrate/20251127000001_create_external_accounts.rb

@@ -0,0 +1,15 @@
+class CreateExternalAccounts < ActiveRecord::Migration[8.0]
+  def up
+    create_table :external_accounts do |t|
+    end
+
+    max_id = Account.maximum(:external_account_id) || 0
+    if max_id > 0
+      execute "INSERT INTO external_accounts (id) VALUES (#{max_id})"
+    end
+  end
+
+  def down
+    drop_table :external_accounts
+  end
+end