feat: complete marketplace module migration (Phase 6)

Migrates marketplace module to self-contained structure:
- Create app/modules/marketplace/services/ re-exporting from existing locations
- Create app/modules/marketplace/models/ with marketplace & letzshop models
- Create app/modules/marketplace/schemas/ with product & import schemas
- Create app/modules/marketplace/tasks/ with 5 Celery tasks:
  - process_marketplace_import - CSV product import
  - process_historical_import - Letzshop order import
  - sync_vendor_directory - Scheduled daily vendor sync
  - export_vendor_products_to_folder - Multi-language export
  - export_marketplace_products - Admin export
- Create app/modules/marketplace/exceptions.py
- Update definition.py with is_self_contained=True and scheduled_tasks

Celery task migration:
- process_marketplace_import, process_historical_import -> import_tasks.py
- sync_vendor_directory -> sync_tasks.py (scheduled daily at 02:00)
- export_vendor_products_to_folder, export_marketplace_products -> export_tasks.py

Backward compatibility:
- Legacy task files now re-export from new locations
- Remove marketplace/letzshop/export from LEGACY_TASK_MODULES

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

app/modules/marketplace/services/__init__.py

@@ -0,0 +1,61 @@
+# app/modules/marketplace/services/__init__.py
+"""
+Marketplace module services.
+
+Re-exports Letzshop and marketplace services from their current locations.
+Services remain in app/services/ for now to avoid breaking existing imports.
+
+Usage:
+    from app.modules.marketplace.services import (
+        letzshop_export_service,
+        marketplace_import_job_service,
+        marketplace_product_service,
+    )
+    from app.modules.marketplace.services.letzshop import (
+        LetzshopClient,
+        LetzshopCredentialsService,
+        LetzshopOrderService,
+        LetzshopVendorSyncService,
+    )
+"""
+
+# Re-export from existing locations for convenience
+from app.services.letzshop_export_service import (
+    LetzshopExportService,
+    letzshop_export_service,
+)
+from app.services.marketplace_import_job_service import (
+    MarketplaceImportJobService,
+    marketplace_import_job_service,
+)
+from app.services.marketplace_product_service import (
+    MarketplaceProductService,
+    marketplace_product_service,
+)
+
+# Letzshop submodule re-exports
+from app.services.letzshop import (
+    LetzshopClient,
+    LetzshopClientError,
+)
+from app.services.letzshop.credentials_service import LetzshopCredentialsService
+from app.services.letzshop.order_service import LetzshopOrderService
+from app.services.letzshop.vendor_sync_service import LetzshopVendorSyncService
+
+__all__ = [
+    # Export service
+    "LetzshopExportService",
+    "letzshop_export_service",
+    # Import job service
+    "MarketplaceImportJobService",
+    "marketplace_import_job_service",
+    # Product service
+    "MarketplaceProductService",
+    "marketplace_product_service",
+    # Letzshop services
+    "LetzshopClient",
+    "LetzshopClientError",
+    "LetzshopCredentialsService",
+    "LetzshopOrderService",
+    "LetzshopVendorSyncService",
+]