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/services/admin_service.py

@@ -37,7 +37,7 @@ from app.modules.marketplace.models import MarketplaceImportJob
 from models.database.platform import Platform
 from models.database.user import User
 from models.database.vendor import Role, Vendor
-from models.schema.marketplace_import_job import MarketplaceImportJobResponse
+from app.modules.marketplace.schemas import MarketplaceImportJobResponse
 from models.schema.vendor import VendorCreate
 
 logger = logging.getLogger(__name__)