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/modules/catalog/schemas/catalog.py

@@ -11,7 +11,7 @@ from datetime import datetime
 from pydantic import BaseModel, ConfigDict, Field
 
 from app.modules.inventory.schemas import InventoryLocationResponse
-from models.schema.marketplace_product import MarketplaceProductResponse
+from app.modules.marketplace.schemas import MarketplaceProductResponse
 
 
 class ProductResponse(BaseModel):