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/__init__.py

@@ -3,10 +3,11 @@
 Marketplace Module - Letzshop integration.
 
 This module provides:
-- Letzshop product sync
-- Marketplace import operations
-- Product catalog synchronization
-- Order import from marketplace
+- Product import from marketplace CSV feeds
+- Order import from Letzshop API
+- Vendor directory synchronization
+- Product export to Letzshop CSV format
+- Scheduled sync tasks
 
 Dependencies:
 - Requires: inventory module (for product management)
@@ -18,8 +19,20 @@ Routes:
 Menu Items:
 - Admin: marketplace-letzshop
 - Vendor: marketplace, letzshop
+
+Usage:
+    from app.modules.marketplace import marketplace_module
+    from app.modules.marketplace.services import letzshop_export_service
+    from app.modules.marketplace.models import MarketplaceProduct
+    from app.modules.marketplace.exceptions import LetzshopClientError
 """
 
-from app.modules.marketplace.definition import marketplace_module
+from app.modules.marketplace.definition import (
+    marketplace_module,
+    get_marketplace_module_with_routers,
+)
 
-__all__ = ["marketplace_module"]
+__all__ = [
+    "marketplace_module",
+    "get_marketplace_module_with_routers",
+]