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

.cursor/hooks/state/continual-learning.json

@@ -1,8 +1,8 @@
 {
   "version": 1,
-  "lastRunAtMs": 1778916916450,
-  "turnsSinceLastRun": 9,
-  "lastTranscriptMtimeMs": 1778916916347.422,
-  "lastProcessedGenerationId": "309e0c67-c6f3-45de-b400-cc4455d26b28",
+  "lastRunAtMs": 1778964093127,
+  "turnsSinceLastRun": 0,
+  "lastTranscriptMtimeMs": 1778964092911.085,
+  "lastProcessedGenerationId": "370e16e5-9bc9-4e07-a658-507f07456acf",
   "trialStartedAtMs": null
 }