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>
Transition from team-based to account-based multi-tenancy (Free/Pro/Team).
Migrations 016-020 create accounts, subscriptions, plan_limits, and
account_invites tables, then migrate existing users and content FKs.
New models: Account, Subscription, PlanLimits, AccountInvite.
Updated models add account_id alongside existing team_id (coexistence
for safe two-PR deployment). Permissions and deps refactored for
account_role instead of is_team_admin.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
