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/core/celery_config.py

@@ -49,13 +49,15 @@ if SENTRY_DSN:
 # TASK DISCOVERY
 # =============================================================================
 # Legacy tasks (will be migrated to modules over time)
-# NOTE: Most subscription tasks have been migrated to app.modules.billing.tasks
-#       The subscription module is kept for capture_capacity_snapshot (will move to monitoring)
+# MIGRATION STATUS:
+# - subscription: MIGRATED to billing module (kept for capture_capacity_snapshot -> monitoring)
+# - marketplace, letzshop, export: MIGRATED to marketplace module
+# - code_quality, test_runner: Will migrate to dev-tools module
 LEGACY_TASK_MODULES = [
-    "app.tasks.celery_tasks.marketplace",
-    "app.tasks.celery_tasks.letzshop",
+    # "app.tasks.celery_tasks.marketplace",  # MIGRATED to marketplace module
+    # "app.tasks.celery_tasks.letzshop",  # MIGRATED to marketplace module
     "app.tasks.celery_tasks.subscription",  # Kept for capture_capacity_snapshot only
-    "app.tasks.celery_tasks.export",
+    # "app.tasks.celery_tasks.export",  # MIGRATED to marketplace module
     "app.tasks.celery_tasks.code_quality",
     "app.tasks.celery_tasks.test_runner",
 ]