docs(plan): self-serve signup & onboarding implementation plans
Adds two phase plans alongside the spec at docs/superpowers/specs/2026-05-05-self-serve-signup-onboarding-design.md: - Phase 1 (backend foundation, 26 tasks across 8 sub-phases A-H): schema migrations, subscription model + new guards, BillingService, Stripe webhook handler extension, OAuth callbacks, email verification auto-send + email-match enforcement, account-invite extensions, GET /billing/state, pilot user backfill. Step-by-step granularity with full code blocks per writing-plans skill. - Phase 2 (frontend + cutover, 21 tasks across 7 sub-phases I-O): Phase-1-deferred endpoints, useBillingStore + hooks + gating components, register redesign + OAuth buttons + accept-invite, welcome wizard, dashboard redesign, pricing page + contact-sales, beta-signup deprecation, cutover. Higher-altitude — defines contracts, acceptance criteria, integration tests; leaves component-detail decisions to implementer. Each phase ends in a mergeable PR. Cutover is gated behind SELF_SERVE_ENABLED + VITE_SELF_SERVE_ENABLED. Execution deferred to a future session. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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