feat: add subscriptions feature capsule - M1 database schema and Stripe client (refs #55)

- Create 4 new tables: subscriptions, subscription_events, donations, tier_vehicle_selections
- Add StripeClient wrapper with createCustomer, createSubscription, cancelSubscription,
  updatePaymentMethod, createPaymentIntent, constructWebhookEvent methods
- Implement SubscriptionsRepository with full CRUD and mapRow case conversion
- Add domain types for all subscription entities
- Install stripe npm package v20.2.0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/package-lock.json

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@@ -7635,6 +7629,26 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
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+    "node_modules/stripe": {
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