feat(story-3.6): complete Stripe subscription tiers — enterprise card, build fix, i18n

Story 3.6: Stripe Subscription Tiers
- Verified all pre-existing billing implementation (API routes, webhook, sync, UI)
- Added Enterprise plan card with contact sales CTA (mailto:sales@momento.app)
- Fixed lib/stripe.ts build error (lazy getStripe() + placeholder default)
- Added enterpriseFeature1-5 i18n keys to all 15 locales
- 22/22 unit tests pass, build succeeds
- Story status: ready-for-dev → review

docs/sprint-status.yaml

@@ -1,5 +1,5 @@
 # generated: 2026-05-14T16:06:50Z
-# last_updated: 2026-05-15T20:00:00Z
+# last_updated: 2026-05-16T21:00:00Z
 # project: Momento
 # project_key: NOKEY
 # tracking_system: file-system
@@ -35,7 +35,7 @@
 # - Dev moves story to 'review', then runs code-review (fresh context, different LLM recommended)
 
 generated: 2026-05-14T16:06:50Z
-last_updated: 2026-05-15T20:00:00Z
+last_updated: 2026-05-16T21:00:00Z
 project: Momento
 project_key: NOKEY
 tracking_system: file-system
@@ -48,10 +48,10 @@ development_status:
   3-3-smart-routing-fallback: done
   3-4-host-pays-session-logic: done
   3-5-secure-byok-management: review
-  3-6-stripe-subscription-tiers: ready-for-dev
+  3-6-stripe-subscription-tiers: review
   epic-3-retrospective: optional
-  epic-4: backlog
-  4-1-gdpr-cookie-consent: backlog
+  epic-4: in-progress
+  4-1-gdpr-cookie-consent: ready-for-dev
   4-2-gdpr-right-to-be-forgotten: backlog
   4-3-data-portability: backlog
   4-4-explicit-ai-consent: backlog