refactor: migrate schemas to canonical module locations

Migrate remaining legacy schemas to their respective modules:

Marketplace module (app/modules/marketplace/schemas/):
- letzshop.py: Letzshop credentials, orders, fulfillment, sync
- onboarding.py: Vendor onboarding wizard schemas

Catalog module (app/modules/catalog/schemas/):
- product.py: ProductCreate, ProductUpdate, ProductResponse

Payments module (app/modules/payments/schemas/):
- payment.py: PaymentConfig, Stripe, transactions, balance

Delete legacy files:
- models/schema/letzshop.py
- models/schema/onboarding.py
- models/schema/product.py
- models/schema/payment.py
- models/schema/marketplace_product.py (re-export)
- models/schema/marketplace_import_job.py (re-export)
- models/schema/search.py (empty)

Update imports across 19 files to use canonical locations.

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

app/api/v1/vendor/payments.py

@@ -15,14 +15,14 @@ from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
 from app.services.vendor_service import vendor_service
 from models.database.user import User
-from models.schema.payment import (
+from app.modules.payments.schemas import (
     PaymentBalanceResponse,
     PaymentConfigResponse,
     PaymentConfigUpdate,
     PaymentConfigUpdateResponse,
     PaymentMethodsResponse,
-    RefundRequest,
-    RefundResponse,
+    PaymentRefundRequest as RefundRequest,
+    PaymentRefundResponse as RefundResponse,
     StripeConnectRequest,
     StripeConnectResponse,
     StripeDisconnectResponse,