[billing] Stripe Tax + per-site tier enforcement

- Enable Stripe Tax for EU VAT compliance (needs EU OSS registration)

- Update `tier-enforcement.ts` to read `websites.tier` instead of `user_profiles.tier`

- Webhook handler to update `websites.tier` on subscription changes

**Files:** `src/lib/tier-enforcement.ts`, `app/api/webhooks/stripe/route.ts`