Cookie-based handoff pattern for existing users in the get-started
flow. After OTP verification detects portal_exists, BFF sets a
short-lived HttpOnly cookie and redirects to the login page, which
shows a password-only form (no second OTP).
Covers splitting the god class into per-path workflow services,
compensating transactions via DistributedTransactionService,
error classification, XState frontend state machine, and
legacy signup flow removal.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
