Billing is inert scaffolding — plan can never be changed

[glennmichael123]

Severity: High (commercialization blocker) · Where: lib/plans.ts; auth.ts:405
Quotas are enforced but user.plan is read-only; there is no payment/checkout flow anywhere.
Fix: Wire a billing provider + plan change; or document self-host-only and remove paid-tier scaffolding.