Replace team_id with account_id across all API endpoints (trees,
categories, tags, steps, step_categories, admin, auth). Add new
accounts and webhooks endpoints. Registration now atomically creates
Account + Subscription, with account_invite_code bypassing the
platform invite gate.
Schemas updated for account_id/account_role. 82 tests passing
including 18 new tests for accounts, subscriptions, and permissions.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
