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>

docs/backend/overview.md

@@ -89,7 +89,7 @@ from fastapi import APIRouter, Depends
 from sqlalchemy.orm import Session
 from app.core.database import get_db
 from app.services.product_service import product_service
-from models.schema.product import ProductCreate, ProductResponse
+from app.modules.catalog.schemas import ProductCreate, ProductResponse
 
 router = APIRouter(prefix="/products")