feat: complete marketplace module self-containment

Migrate marketplace module to self-contained structure:

- routes/api/admin.py - Admin API endpoints
- routes/api/vendor.py - Vendor API endpoints
- routes/pages/ - Page routes (placeholder)
- models/letzshop.py - Letzshop model
- models/marketplace_import_job.py - Import job model
- models/marketplace_product.py - Product model
- models/marketplace_product_translation.py - Translation model
- schemas/marketplace_import_job.py - Import job schemas
- schemas/marketplace_product.py - Product schemas
- locales/ - Translations (en, de, fr, lu)

Removed legacy route files replaced by api/ structure.
Updated __init__.py files to use new structure.

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

app/modules/marketplace/definition.py

@@ -14,14 +14,14 @@ from models.database.admin_menu_config import FrontendType
 
 def _get_admin_router():
     """Lazy import of admin router to avoid circular imports."""
-    from app.modules.marketplace.routes.admin import admin_router
+    from app.modules.marketplace.routes.api.admin import admin_router
 
     return admin_router
 
 
 def _get_vendor_router():
     """Lazy import of vendor router to avoid circular imports."""
-    from app.modules.marketplace.routes.vendor import vendor_router
+    from app.modules.marketplace.routes.api.vendor import vendor_router
 
     return vendor_router