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

backend/src/v1/payments/dependencies.py

@@ -5,6 +5,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from core.db import get_db
 from v1.payments.apple_verifier import AppleJwsVerifier
+from v1.payments.creem_client import CreemClient
+from v1.payments.creem_service import CreemService
 from v1.payments.repository import PaymentRepository
 from v1.payments.service import PaymentService
 from v1.points.repository import PointsRepository
@@ -19,3 +21,14 @@ def get_payment_service(session: AsyncSession = Depends(get_db)) -> PaymentServi
         points_repo=points_repo,
         verifier=verifier,
     )
+
+
+def get_creem_service(session: AsyncSession = Depends(get_db)) -> CreemService:
+    payment_repo = PaymentRepository(session)
+    points_repo = PointsRepository(session)
+    client = CreemClient()
+    return CreemService(
+        payment_repo=payment_repo,
+        points_repo=points_repo,
+        client=client,
+    )