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/models/__init__.py

@@ -0,0 +1,42 @@
+# app/modules/marketplace/models/__init__.py
+"""
+Marketplace module models.
+
+Re-exports marketplace and Letzshop models from the central models location.
+Models remain in models/database/ for now to avoid breaking existing imports.
+
+Usage:
+    from app.modules.marketplace.models import (
+        MarketplaceProduct,
+        MarketplaceImportJob,
+        VendorLetzshopCredentials,
+        LetzshopHistoricalImportJob,
+    )
+"""
+
+from models.database.marketplace_product import MarketplaceProduct
+from models.database.marketplace_product_translation import MarketplaceProductTranslation
+from models.database.marketplace_import_job import MarketplaceImportJob
+from models.database.letzshop import (
+    # Letzshop credentials and sync
+    VendorLetzshopCredentials,
+    LetzshopFulfillmentQueue,
+    LetzshopVendorCache,
+    LetzshopSyncLog,
+    # Import jobs
+    LetzshopHistoricalImportJob,
+)
+
+__all__ = [
+    # Marketplace products
+    "MarketplaceProduct",
+    "MarketplaceProductTranslation",
+    # Import jobs
+    "MarketplaceImportJob",
+    "LetzshopHistoricalImportJob",
+    # Letzshop models
+    "VendorLetzshopCredentials",
+    "LetzshopFulfillmentQueue",
+    "LetzshopVendorCache",
+    "LetzshopSyncLog",
+]