Account.owner_id and User.account_id are both NOT NULL, creating a
circular dependency that prevents inserting either row first. Fix by:
1. Making owner_id nullable (set immediately after user creation)
2. Creating Account before User, then setting owner_id after flush
3. Removing NOT NULL enforcement on owner_id in migration 020
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>