Implement subscription checkout flow in onboarding

[kivo360]

Status: Backend Complete, Frontend Integration Needed

The backend subscription checkout is fully implemented:

• POST /checkout/credits — Stripe Checkout for credit purchase
• POST /checkout/lifetime — lifetime purchase checkout
• POST /subscriptions/checkout — subscription checkout session
• 5 subscription tiers: Free, Pro ($299/mo), Team ($999/mo), Enterprise (custom), Lifetime ($499 one-time)
• Stripe webhook handler processes checkout.session.completed events
• Usage quota enforcement via BillingService.check_and_reserve_workflow()

Remaining Work

• Wire frontend onboarding step to backend checkout endpoints
• Add subscription tier selection UI in onboarding flow
• Add billing management page (Stripe Customer Portal redirect exists)
• Test end-to-end checkout flow with Stripe test keys

Context

• Backend billing routes: backend/omoi_os/api/routes/billing.py (2400+ lines)
• Subscription service: backend/omoi_os/services/subscription_service.py
• Stripe service: backend/omoi_os/services/stripe_service.py