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/routes/pages/__init__.py

@@ -0,0 +1,9 @@
+# app/modules/marketplace/routes/pages/__init__.py
+"""
+Marketplace module page routes (HTML rendering).
+
+Provides Jinja2 template rendering for marketplace management.
+Note: Page routes can be added here as needed.
+"""
+
+__all__ = []