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/onboarding.py

@@ -21,7 +21,7 @@ from app.core.database import get_db
 from app.services.onboarding_service import OnboardingService
 from app.tasks.letzshop_tasks import process_historical_import
 from models.database.user import User
-from models.schema.onboarding import (
+from app.modules.marketplace.schemas import (
     CompanyProfileRequest,
     CompanyProfileResponse,
     LetzshopApiConfigRequest,