feat: integrate CREEM web payment for credits purchase

Replace abandoned iOS App Store route with CREEM Merchant of Record
payment integration for web-based credits purchase.

Backend changes:
- Add CreemClient for CREEM API communication
- Add CreemService for checkout creation and webhook handling
- Add creem_transactions table for payment tracking
- Fix webhook payload parsing (id, order.id, customer.id structure)
- Integrate with existing points ledger system

Frontend changes:
- Display dynamic prices from CREEM API
- Support decimal price formatting (e.g., $1.00)
- Add checkout flow with redirect to CREEM hosted page

web/src/lib/api-routes.ts

@@ -16,6 +16,9 @@ export const API_ROUTES = {
     balance: '/api/v1/points/balance',
     packages: '/api/v1/points/packages',
   },
+  payments: {
+    creemCheckout: '/api/v1/payments/creem/checkouts',
+  },
   notifications: {
     list: '/api/v1/notifications',
     unreadCount: '/api/v1/notifications/unread-count',