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>
